DEVELOPMENT OF THE OVUM.

Membrana decidua. The earliest trace of impregnation which is to be observed in the cavity of the uterus, and even before the ovum has reached it, is the presence of a soft humid paste-like secretion, with which the cavity of the uterus is covered, and which is furnished by the secreting vessels of its lining membrane. This is the membrana decidua of Hunter: properly speaking, it should be called the maternal membrane, in contra-distinction to the chorion and amnion, which, as belonging peculiarly to the foetus, are called the foetal membranes.[17]

Although at first in a semi-liquid state, it soon becomes firmer and more compact, assuming the character of a membrane: it appears to be nothing else than an effusion of coagulable lymph on the internal surface of the uterus, having “scarcely a more firm consistence than curd of milk or coagulum of blood.” (Hunter, op. cit. p. 54.) Hence, although much thicker than the other membranes, it is weaker; it is also much less transparent.

It is not of an equal thickness, being considerably thicker in the neighbourhood of the placenta than elsewhere; inferiorily, and especially near the os uteri, it becomes thinner: during the first weeks of pregnancy it is much thicker than afterwards, becoming gradually thinner as pregnancy advances, until it is not half a line in thickness. In the earlier months its external surface is rough and flocculent, but afterwards it becomes smoother as its inner surface was at an earlier period.

It is much more loosely connected with the uterus during the first months of pregnancy than afterwards, and this is one reason why premature expulsion of the ovum is more liable to take place at this period than during the middle and latter part of utero-gestation. It is more firmly attached to the uterus in the vicinity of the placenta than any where else, which is owing to the greater number of blood-vessels it receives from the uterus at this point; whereas commonly “it has no perceptible blood-vessels at that part which is situated near the cervix uteri,” (Ibid.,) this portion being much more loosely connected with the uterus. The course which the decidual vessels take on coming from the inner surface of the uterus is admirably adapted to render the attachment of this membrane to it as firm as possible.

Upon examining the lining membrane of the uterus at a very early period, when the decidua was still in a pulpy state, Professor v. Baer observed[18] that its villi, which in an unimpregnated state are very short, were remarkably elongated: between these villi, and passing over them, was a substance, not organized but merely effused, and evidently the membrana decidua at an extremely early age. The uterine vessels were continued into this substance, and formed a number of little loops round the villi, thus anastomosing with each other. On account of this reticular distribution it was impossible to distinguish arteries from veins; there is evidently the same relation between the uterus and the decidua as between an inflamed surface and the coagulable lymph effused upon it.

Professor v. Baer considers that at a later period the connexion between the decidua and mucous membrane becomes so intimate, that it is impossible to separate the former without also separating the latter from the fibrous tissue of the uterus. This, we apprehend, is the stratum which, as Dr. Hunter observes, “is always left upon the uterus after delivery, most of which dissolves and comes away with the lochia.” He does not appear to have been fully aware of the close connexion between the decidua and lining membrane of the uterus, although he evidently observed the fact from the following sentence: “in separating the membranes from the uterus we observe that the adhesion of the decidua to the chorion, and likewise its adhesion to the muscular fibres of the uterus, is rather stronger than the adhesion between its external and internal stratum, which, we may presume, is the reason that in labour it so commonly leaves a stratum upon the inside of the uterus.” According to the observations of Dr. Montgomery, a great number of small cup-like elevations may be seen upon the external surface of the decidua vera, “having the appearance of little bags, the bottoms of which are attached to, or embedded in, its substance; they then expand or belly out a little, and again grow smaller towards their outer or uterine end, which, in by far the greater number of them, is an open mouth when separated from the uterus: how it may be while they are adherent, I cannot at present say. Some of them which I have found more deeply embedded in the decidua were completely closed sacs. They are best seen about the second or third month, and are not to be found at the advanced periods of gestation.”[19]

Decidual cotyledons. From Dr. Montgomery.

The membrana decidua does not envelope the ovum with a single covering, but forms a double membrane upon it, somewhat like a serous membrane; in fact, the descent of the ovum through the Fallopian tube is very similar to that of the testicle through the inguinal canal into the scrotum. The ovum pushes before it that portion of the decidua which covers the uterine extremity of the Fallopian tube, and enters the cavity of the uterus, which is already lined with decidua, covered by the protruded portion which forms the decidua reflexa. It must not be supposed that this reflexion of the decidua is completed as soon as the ovum enters the uterine cavity; the ovum usually remains at the mouth of the Fallopian tube, from which it has emerged, covered by the plastic mass of soft decidua, and the reflexion of this membrane will take place in proportion as the ovum gradually increases in size. The external layer of decidua is called decidua vera; the internal or reflected portion is called the decidua reflexa, having received this appellation from its discoverer, Dr. Hunter. These membranes would, as Dr. Baillie has correctly observed, be more correctly named the decidua uteri and decidua chorii: the decidua chorii or reflexa is reflected inwardly from above downwards; it is connected on its inner surface with the chorion: externally it is unattached, whereas, the decidua uteri or vera is unconnected on its inner surface, but attached to the uterus externally.

The membrana decidua differs in its arrangement from that of a serous membrane, inasmuch, as it is not only reflected so as to cover the chorion, but at the point of reflexion it is continued over the chorion externally, where it forms the placenta, so that the chorion is enclosed in all directions by the decidua: this latter portion, however, is not formed till about the middle of pregnancy. The decidua uteri or vera does not extend farther than the os uteri internum, which is filled up by the plug of tough gelatinous substance above described; the decidua chorii or reflexa, from its forming the outer covering of the chorion, of course passes over the os uteri.

Membrana decidua.

The lower orifice corresponds to the os uteri,
the two upper ones to the Fallopian tubes.
From Dr. Hunter.

According to Mr. John Hunter, the decidua vera is continued some little way into the Fallopian tubes, more especially, on that side where the corpus luteum has been formed; it is perforated at the points where the Fallopian tubes enter, as well as at the os uteri, a fact which is beautifully shown in Dr. Hunter’s last plate: but this does not continue long, for, as Mr. John Hunter observes, the inferiour opening becomes closed in the first month, and, according to Lobstein’s observations, the openings of the Fallopian tubes are closed after the second month. “Where the decidua reflexa is beginning to pass over the chorion, there is, at an early period of pregnancy, an angle formed between it and the decidua, which lines the uterus; and here the decidua is often extremely thin and perforated with small openings so as to look like a piece of lace.

“In proportion as pregnancy advances, the decidua reflexa becomes gradually thinner and thinner, so that at the fourth month it forms an extremely fine layer covering the chorion; it comes at the same time more and more closely in contact with the decidua, which lines that part of the uterus to which the placenta is not fixed, till at length they adhere together.”[20] That portion of the decidua which passes between the placenta and uterus during the latter half of gestation, is called the placental decidua, the description of which will be given with that of the placenta.

To Dr. W. Hunter are we indebted for the first correct description of the decidua; indeed, so excellent is it, that the membrane has been called after him, the decidua of Hunter. Although he was the undoubted discoverer of the reflexa, the existence of the decidua was distinctly noticed by Burton, in 1751. In stating the post mortem examination of a woman, who died undelivered at the full time of pregnancy, he says, “Upon wiping the inside of the uterus very gently with a sponge, there seemed to be pieces of a very tender thin transparent membrane adhering to it in such parts of the uterus where the placenta did not stick to it; but as the womb was somewhat corrupted, and the membrane so very tender, we could not raise any bulk of it so as to be certain what it was.” (Burton’s Midwifery.)

The decidua seems chiefly intended to form the maternal part of the placenta: (see Placenta:) hence in all those quadrupeds when the maternal part of the placenta is permanently appended to the internal surface of the uterus, no decidua is found.

Having described the maternal membranes of the ovum, we come now to the membranes which form the parietes of the ovum. These are called the foetal membranes, for they are essentially connected with the origin of the foetus itself. They are the chorion and the amnion; besides which, there are two others that require notice, viz. the vesicula umbilicalis and allantois.

Chorion. The chorion is the proper covering of the ovum, and corresponds to the membrane lining the shell of an egg, in oviparous animals. It is a thin and transparent membrane, and presents on its external surface a ragged tufted appearance, being covered externally with groups of arborescent villous processes, which after a time unite into trunks to form the umbilical vessels, which, according to Lobstein’s observations, are merely veins during the early period of gestation. These loose tufts of venous radicles appear to absorb nourishment for the ovum, much in the same manner as the roots of a plant. Although the chorion is so thin and transparent, it consists nevertheless of two laminÆ or layers, between which the villi, which produce this shaggy appearance, take their course. Although the chorion on its external surface is nothing but a net-work of villi, which in process of time become vascular, anatomists have been unable to detect blood-vessels in the structure of the membrane itself. Its vascularity, however, has been asserted chiefly on the ground of the known vascularity of the decidua, it being supposed that the vessels of the decidua penetrate into the chorion. The chorion, however, belongs so essentially and exclusively to the foetus, that it appears extremely improbable that any maternal vessels should ramify in its structure for the purposes of its nourishment and growth, and the more so when we reflect that the nutrition of the foetus itself at this early period is obtained in so different a manner. It is, moreover, extremely difficult to distinguish between the venous absorbing radicles of the chorion, which form the early rudiments of the umbilical vessels, and any vessels which may take their course in the structure of the membrane itself; and the more we consider the relation between the chorion and the decidua, the less are we inclined to accept Meckel’s explanation of the vascularity of the chorion, viz. that the vessels of the decidua have the same relation to those of the chorion as the blood-vessels of the maternal part of the placenta have to those of the foetal part.

Neither nerves nor lymphatics have been discovered in the structure of the chorion, unless, indeed, those white filaments, which are observed here and there about the edge of the placenta, perform the office of lymphatics. This has been hinted at by Dr. Hunter, where he says, “these are the remains of those shaggy vessels which shoot out from the chorion in a young conception, and give the appearance of the ovum being altogether surrounded by the placenta at that time. With a magnifying glass, they appear to be transparent ramifying vessels, which run in corresponding furrows upon the internal surface of the decidua, and a good deal resemble lymphatics.” (W. Hunter, op. cit. p. 53.)

The chorion undergoes various changes during the different periods of pregnancy, and forms a very important part of the physiology of utero-gestation. Its thickness, which in the earlier months of pregnancy is more considerable than afterwards, at this period is uniform in every part of the ovum: its external surface covered with those villous prolongations which have already been alluded to. In the second month of pregnancy these become larger, and much more arborescent; after the third month a considerable portion of them gradually disappears, generally from below upwards, so that the greater part of its external surface becomes nearly smooth, except at that point where the umbilical cord has its origin, at which spot the villous prolongations become more developed, and unite to form the umbilical vessels. This part of the chorion, together with the corresponding portion of the membrana decidua, forms a flat circular mass, which at the end of pregnancy covers nearly one-third of the surface of the ovum, and constitutes the placenta or after-birth. At this point the chorion, which forms its inner surface, is considerably thicker than elsewhere.

At the commencement of pregnancy the chorion is but loosely connected with the decidua, but by degrees it becomes so closely connected by fibres, which are the remains of the little vascular prolongations, especially where these two membranes combine to form the placenta, that in the latter months of pregnancy, they can scarcely, if at all, be separated.

For the more minute consideration of the formation, development, and functions of the chorion, we must refer to the description of the placenta and foetus.

Amnion. The amnion is the inner membrane of the ovum. It is transparent, and of great tenuity, “yet its texture is firm, so as to resist laceration much more than the other membranes.” (W. Hunter, op. cit. p. 50.) It is loosely connected with the chorion on its external surface, except when this membrane unites with the decidua to form the placenta at which spot it adheres to the chorion much more firmly. Its inner surface, which is in immediate contact with the liquor amnii, is very smooth; whereas externally, from being connected with the chorion by an exceedingly fine layer of cellular tissue, its surface is not so smooth. Dr. W. Hunter considers that this intervening tissue, is a gelatinous substance: it seems, however, to possess too much elasticity for such a structure; and, from the reticular appearance which it generally presents upon the membranes to which it adheres, we are inclined to adopt the opinion of Meckel in considering it cellular. “In the very early state of an ovum the amnium forms a bag, which is a good deal smaller than the chorion, and, therefore, is not in contact with it.” (Ibid. p. 75:) hence, therefore, a space is formed between the two membranes which is filled with a fluid called the liquor amnii spurius, or more correctly the liquor allantoidis. “In the course of some weeks, however, it comes nearly into contact with the chorion, and through the greater part of pregnancy the two membranes are pretty closely applied to each other.” (Ibid.) Lobstein, in his admirable Essai sur la Nutrition du Foetus, observes, that the membranes continues separate from each other so late as the third and fourth month. Cases every now and then occur where a considerable quantity of fluid is found between the chorion and amnion in labour at the full period of pregnancy.

We shall defer the minute description of the amnion and its relations, during the very early periods of utero-gestation, until we describe the embryo. The amnion is reflected upon the umbilical cord at its insertion into the placenta, envelopes the umbilical vessels, the external covering of which it forms, and is continued to the anterior surface of the child’s abdomen, passing into that projecting portion of the skin which forms the future navel.

Blood-vessels and nerves have not as yet been discovered in the structure of the amnion, but Meckel considers it extremely probable that the fine layer of cellular tissue by which it is connected with the chorion contains vessels for its nutrition.

Liquor amnii. The amnion contains a fluid known by the name of liquor amnii. In the earlier months of pregnancy it is nearly, if not quite transparent; as pregnancy advances it becomes turbid, containing more or less of what appears to resemble mucus: it has a distinctly saline taste; its specific gravity is rather more than that of water. Its relative and absolute quantity vary considerably at different periods of pregnancy: thus the relative weight of liquor amnii to that of the foetus is very considerable at the beginning of pregnancy, at the middle they are nearly equal, but towards the end, the weight of fluid to that of the child, diminishes considerably, so that during the last weeks of pregnancy it scarcely equals a pound, and seldom more than eight ounces, whereas the medium weight of the child is usually between six and seven pounds: the quantity, however, varies considerably, sometimes amounting to several quarts. In the early months the absolute quantity increases, so that between the third and fourth months it sometimes equals as much as thirty-six ounces. Chemically it consists chiefly of water, a small quantity of albumen and gelatine, a peculiar acid called amniotic, with a little muriate of soda and ammonia, and a trace of phosphate of lime.

The source of the liquor amnii is still unknown. Dr. Burns asserts that “it is secreted from the inner surface of the membrane by pellucid vessels,” but as he confesses that “these have never been injected or traced to their source (Principles of Midwifery, by J. Burns, M. D. p. 222.,) little weight can be attached to such a view.” Meckel considers (Handbuch der Menschlichen Anatomie, vol. iv. p. 707,) that the greater part of it, especially in the early months, is a secretion from the maternal vessels, but that afterwards, as pregnancy advances, it becomes mingled with the excretions of the foetus. It appears to be a means of nourishment to the foetus during the first part of pregnancy, from the fact that it contains more nutritious matter in the early than in the latter months, since at that time a considerable coagulation is produced by alcohol, &c. The disappearance of this coagulable matter of the liquor amnii, towards the end of pregnancy, may be attributed to its having been absorbed at an earlier period, and to the process of nutrition being now carried on by other means. Besides being a source of nourishment to the foetus, it serves many useful purposes; it secures the foetus against external pressure or violence, and supports the regular distension of the uterus; on the other hand it diminishes and equalises the pressure of the foetus upon the uterus; during labour by distending the membranes into an elastic cone, it materially assists to dilate the os uteri; it also serves to lubricate and moisten the external passages.

Placenta. The placenta is formed essentially by the chorion and decidua; it is a flat, circular, or more or less oval mass, soft, but becoming firmer towards its edge. It is the most vascular part of the ovum, and by which it is connected most intimately with the uterus. Its longest diameter is generally about eight, its shortest about six inches; its greatest thickness is at that spot where the umbilical cord is inserted, which is usually about the middle of the placenta, although it occasionally varies considerably in this respect, the cord coming off sometimes at the edge. The placenta, as ordinarily seen after labour, is barely an inch in its thickest part, but when filled with blood or injection it swells very considerably, and is then little short of two inches. It is generally attached to the upper part of the uterus in the neighbourhood of one of the Fallopian tubes, and more frequently on the left side than on the right; its inner or foetal surface is smooth, being covered by the chorion, which at this part is much thicker.

The placenta cannot be distinguished from the other parts of the ovum until the end of the second month, at which period it covers nearly half the surface of the ovum, gradually diminishing in relative size, but increasing in thickness and absolute bulk up to the full period of utero-gestation. It forms a spongy vascular mass, its uterine surface being divided unequally into irregular lobes called cotyledons.

The uterine surface of a full-grown placenta is covered by a pulpy membrane, resembling in structure the decidua which covers the chorion, and of which it seems to be a continuation. This is always found present at the end of pregnancy: it covers the lobes of the uterine surface of the placenta, descending into the sulci which runs between them: in some parts it is thicker than in others, especially where it is connected with, or in fact becomes, the decidua of the chorion or decidua reflexa. This membrane, which has been called the placenta decidua, is pretty firmly attached to the vessels of the placenta, so as not to be separated without rupture; but by maceration, its texture is more or less destroyed, so that we may easily distinguish the extremities of these vessels. “This decidua, or uterine portion of the placenta,” says Dr. Hunter, “is not a simple thin membrane expanded over the surface of the part: it produces a thousand irregular processes, which pervade the substance of the placenta as deep as the chorion or inner surface; and are every where so blended and entangled with the ramifications of the umbilical system, that no anatomist will perhaps be able to discover the nature of their union. While these two parts are combined, the placenta makes a pretty firm mass, no part of it is loose or floating; but when they are carefully separated, the umbilical system is evidently nothing but loose floating ramifications of the umbilical vessels, like that vascular portion of the chorion, which makes part of the placentula in a calf; and the uterine part is seen shooting out into innumerable floating processes and rugÆ, with the most irregular and minutely subdivided cavities between them that can be conceived. This part answers to the uterine fungus in the quadrupeds: it receives no vessels demonstrable by the finest injection from those of the navel string; yet it is full of both large and small arteries and veins: these are all branches of the uterine vessels, and are readily filled by injecting the arteries and veins of the uterus, and they all break through in separating the placenta from the uterus, leaving corresponding orifices on the two parted surfaces.” (Hunter, op. cit. p. 42.)

According to Lobstein’s observations, although this membrane appears to be a continuation of the decidua which covers the chorion, it nevertheless does not exist during the earlier months. During the first months of pregnancy the placenta does not present a solid mass, with its uterine surface covered with projecting lobuli, as it does at the full term of pregnancy; but the vessels of which it is composed (foetal) are loose and floating, as if it had been subjected to maceration. It has been supposed, that this irregular lobulated appearance of the uterine surface of the placenta was produced at the moment of its separation from the uterus during labour; this, however, is not the case, for Lobstein having opened the uterus of a woman who died in the fifth month of pregnancy, and separated the placenta with great care, found these lobular prominences, although not yet covered by the membrane of which we have just spoken. Wrisberg, professor of anatomy at GÖttingen, considered that this membrane was distinct from the decidua reflexa, since with care the two membranes can be easily separated.

Uterine surface of the Placenta.

In examining the uterine surface of a full grown placenta it is necessary to place it upon something convex, in order that it may resemble, as nearly as possible, the form which it had when attached to the concave surface of the uterus; the cotyledons are thus rendered prominent and separated from each other; the sulci, which run between them, are wide and gaping: whereas, when the placenta is laid upon a flat surface, its cotyledons are closely pressed together, and the sulci more or less completely concealed. On minute examination of these sulci a number of openings may be observed, varying in size and shape, but usually more or less oval, their edges distinct, smooth, and thin; on directing a strong light into some of the larger ones a number of smaller apertures may be observed opening into them, in much the same way as is observed when looking down a large vein. Some of these canals do not immediately lead to smaller orifices as above described, but open at once into an irregular-shaped cell or cavity, in the parietes of which numerous small apertures may be observed, through which blood oozes when the adjacent parts of the placenta are slightly pressed upon. Besides these openings at the bottom of the interlobular sulci, others may be seen here and there upon the cotyledons; these are generally smaller, their edges thicker, and in most instances they are round; but they are not so invariably met with as the openings between the cotyledons, these lobular projections being sometimes very thickly covered with placental decidua. The openings observed on the uterine surface of the placenta correspond to the mouths of the uterine veins and arteries, which, in the unimpregnated state, open into the cavity of the uterus, but which now, by means of the decidua, convey maternal blood to and from the placenta. “Any anatomist,” says Dr. W. Hunter, “who has once seen and understood them, can readily discover them upon the surface of any fresh placenta; the veins, indeed, he will find have an indistinct appearance from their tenderness and frequent anastomoses, so as to look a good deal like irregular interstitial void spaces: the arteries which generally make a snake-like convolution or two, on the surface of the placenta, and give off no anastomosing branches, are more distinct.” (Hunter, op. cit. p. 46.) From the observations of Messrs. Mayo and Stanley, and from their examination of the original preparations in the Hunterian museum at the College of Surgeons, London, illustrating this subject, it appears that, in all probability, most of the large thin-edged apertures at the bottom of the interlobular sulci are connected with the uterine veins; whereas, the smaller orifices, the margins of which are thicker, and which are chiefly observed upon the cotyledons, are continuations of the uterine arteries.

These openings were also pointed out by the late Dr. Hugh Ley, in describing the post mortem examination of a woman who had died at the full term undelivered (Med. Gaz. June 1, 1833:) “The uterine surface (of the placenta) thus detached from the uterus, exhibited its lobules with their intersecting sulci, even more distinctly than they are seen in the uninjected placenta; and in several parts there could be perceived, with the naked eye, small apertures of an oval form, with edges perfectly smooth, regularly defined, and thicker, as well as more opaque, than the contiguous parts which they penetrated.” The communication between the openings of the placental cells, and the mouths of the uterine veins and arteries, which convey their blood to the placenta, as before observed, is effected by means of the placental decidua. The connecting portion of canal is of a flattened shape, runs obliquely between the uterus and placenta, and appears to be formed entirely of decidua. The manner in which the arteries pass to the placenta is very different to that of the veins: “the arteries,” as Dr. W. Hunter observes, “are all much convoluted and serpentine; the larger, when injected, are almost of the size of crow-quills: the veins have frequent anastomoses.” Mr. J. Hunter has described this point more minutely, and gives still more precise notions of the manner in which the arteries pass to the placenta. “The arteries of the uterus which are not immediately employed in conveying nourishment to it, go on towards the placenta, and, proceeding obliquely between it and the uterus, pass through the decidua without ramifying: just before they enter the placenta, making two or three close spiral turns upon themselves, they open at once into its spongy substance, without any diminution of size, and without passing beyond the surface as above described.

The intention of these spiral turns would appear to be that of diminishing the force of the circulation as it approaches the spongy substance of the placenta, and is a structure which must lessen the quick motion of the blood in a part where a quick motion of this fluid was not wanted. The size of these curling arteries at this termination is about that of a crow’s quill. The veins of the uterus appropriated to bring back the blood from the placenta, commence from this spongy substance by such wide beginnings as are more than equal to the size of the veins themselves. These veins pass obliquely through the decidua to the uterus, enter its substance obliquely, and immediately communicate with the proper veins of the uterus; the area of those veins bear no proportion to their circumference, the veins being very much flattened.”[21]

On examining these vessels in an injected uterus to which the placenta is attached, we shall therefore find that all traces of a regular canal or tube are suddenly lost upon their entering the placenta; each vessel (whether artery or vein) abruptly terminating in a spongy cellular tissue. If a blow-pipe be introduced into a piece of sponge, we shall have a very simple but correct illustration of the manner in which the uterine blood circulates through the placenta. The cell into which each vessel immediately opens is usually much larger than the rest, so that when the cellular structure of the placenta is filled with wax, a number of irregular nodules[22] are found continuous with these vessels and passing into an infinity of minute granules, which are merely so many casts of smaller cells. That this cellular tissue pervades the whole mass of the placenta, and communicates freely with the uterine vessels by which it is filled with blood, is proved by repeating a very simple experiment of Dr. Hunter, viz. “if a blow-pipe be thrust into the substance of the placenta any where, the air which is blown into the cellular part opens, and rushes out readily by, the open mouths both of the arteries and veins.” (Hunter, op. cit. p. 46.) That it also envelopes the umbilical vessels of the cord is shown by the fact, that if a pipe be inserted beneath the outer covering of the cord near to its insertion into the placenta, we shall be able to “fill the whole placenta uniformly in its cellular part, and likewise all the venous system of the uterus and decidua, as readily and fully as if we had fixed the pipe in the spermatic or hypogastric vein; so ready a passage is there reciprocally between the cells of the placenta and the uterine vessels.” (Ibid. p. 47.)

The maternal portion of the placenta therefore consists of a spongy cellular tissue, which is filled by the uterine vessels, and also of those trunks which pass through the decidua, and which form the communication between these vessels and the placental cells.

Foetal surface of the placenta.

The foetal surface of the placenta is smooth and glossy, being covered by the amnion and chorion; it is much harder than the uterine surface, and is streaked over by the larger branches of the umbilical vein and arteries, which radiate irregularly from the point where the cord is inserted; and which pass beneath the amnion, and between the two layers of which the chorion is composed, to which they are intimately connected. These vessels supply the various lobuli of which the placenta is composed, so that each lobulus receives at least one of these branches; for, although the umbilical cord consists of two arteries and one vein, this arrangement does not continue into the body of the placenta. “Every branch of an artery,” as Dr. Hunter observes, “is attended with a branch of a vein: these cling to one another, and frequently in the substance of the placenta entwine round one another, as in the navel string.” (Ibid. p. 40.) Each cotyledon receives its own vessels, so that the vessels of one cotyledon have no direct communication with those of the adjacent ones, as proved by Wrisberg’s examinations; for if we inject the vessel or vessels of one of these lobuli, the injection will not pass into those of the others. When the vessels have reached the cotyledons, they are divided and subdivided ad infinitum; they are connected together by a fine cellular membrane, which may be very easily removed by maceration, and then they may be seen ramifying in the most beautiful and delicate manner possible; the main branches having no communication or anastomosis with each other.

The umbilical arteries anastomose freely with each other upon the foetal surface of the placenta, before dividing into the branches above-mentioned; hence, if an injection be thrown into one umbilical artery it will return almost immediately by the other; but if this be tied also, the injection, after a time, will return by the umbilical vein, but not until all the vessels of the placenta have been filled, proving that there is a free passage of blood from the arteries into the veins.

From these remarks, founded chiefly on the admirable observations of the Hunters, and repeated examinations of the placenta, which we have made with the greatest care and impartiality, it may be stated with confidence, that the placenta consists of two portions—a maternal and a foetal. The maternal portion consists, as we have before observed, of a spongy cellular tissue; and also of those trunks which pass through the decidua, and which form the communication between the uterine vessels and the placental cells. The foetal part is formed by the ramifications of the umbilical vessels: “that each of those parts has its peculiar system of arteries and veins, and its peculiar circulation, receiving blood by its arteries, and returning it by its veins; that the circulation through these parts of the placenta differs in the following manner: in the umbilical portion the arteries terminate in the veins by a continuity of canal; whereas, in the uterine portion there are intermediate cells into which the arteries terminate, and from which the veins begin.” (Hunter, op. cit. p. 48.)

Although various observations and anatomical injections show that to a certain degree, there is a communication between the uterus and the placenta, inasmuch as the blood of the former is received into the sinuses or cells of the latter, we possess no proof that the blood can pass from these sinuses into the umbilical vessels: on the contrary, every thing combines to prove that the circulation of the foetus is altogether independent of that of the mother. We know from daily experience that in labour at the full term of pregnancy, the placenta is easily expelled from the uterus: that, upon examining the surface which had been attached to the uterus we find no laceration, and that a discharge of more or less blood takes place for some days afterwards. We know, also, that when the placenta becomes detached from the uterus during the progress of gestation, it is followed by a considerable hemorrhage, which greatly endangers the life of the mother. These facts prove that there is a circulation of uterine blood in the placenta, which is destroyed upon its being separated from the uterus. That this uterine circulation in the placenta is unconnected with the circulation of foetal vessels in the placenta is proved by the fact first pointed out by Wrisberg, viz. that, where the mother has died from loss of blood, and the maternal vessels therefore drained of their contents, those of the foetus have been full of blood. Still farther to illustrate this fact, he killed several cows big with calf, by a large wound through the heart or great vessels, so as to ensure the most profuse and sudden loss of blood possible, and never found that the vessels of the calf were deprived of blood, although those of the mother were perfectly empty; moreover, no anatomist has ever yet succeeded in making injections pass from the foetal into the uterine vessels, or vice versÂ. Lobstein has mentioned a mode of illustrating this fact (Essai sur la Nutrition du Foetus,) which is both simple and striking. Upon examining the uterine surface of a placenta which has been expelled at the full term, it presents the appearance of a spongy mass gorged with blood, which may be removed by washing or maceration, and if a placenta thus prepared be injected, the fluids will pass with the greatest facility from the umbilical arteries into the umbilical vein, but not one drop into its cellular structure; it is evident, therefore, that the blood which had filled the intervals between the vessels, and which had been removed by washing and maceration, could not have belonged to the foetus, but must have come from the mother; for if any of the vessels had been ruptured the injection would not have succeeded.

In concluding these observations upon the placenta, we may briefly state, that there is the same relation between the umbilical vessels and the maternal blood, which fills the placental cells, as there is between the branches of the pulmonary artery, and the air which fills the bronchial cell.[23]Umbilical cord. The umbilical cord, funis, or navel string, is a vascular rope extending between the foetus and placenta, by which they are connected together. It usually arises, as we have before observed, from about the middle of the placenta, and terminates at the umbilical ring of the foetus; it consists of two umbilical arteries and one umbilical vein; the former conveying the blood from the common iliac arteries of the foetus to the cotyledons of the placenta; the latter formed by the union of the collected umbilical veins, on the inner surface of the placenta, and returning this blood to the foetus. In the early periods of pregnancy it also consists of the duct and vessels of the vesicula umbilicalis, the urachus, and more or less of the intestinal canal. The umbilical cord does not present the same form or appearance at every period of gestation; the younger the embryo, the shorter and thicker is the cord; in fact, there are no traces whatever of a cord at first, the embryo adhering, by its lower or caudal extremity, directly to the membranes. By the fifth or sixth week it becomes visible; at this early period the vessels of which it is composed pass from the foetus in a straight direction, but as pregnancy advances they become more or less spiral, winding round each other, and usually from left to right: according to Meckel, they take the opposite direction much less frequently, viz. in the proportion of one to nine.

The vessels of the umbilical cord are imbedded in a thick viscid substance; upon minute examination, it will be found to consist of a very fine cellular tissue, containing an albuminous matter which slowly exudes, when pressed between the fingers. This cellular tissue itself may be demonstrated by the inflation of air or injection with mercury: it seems to accompany the umbilical vessels as far as the posterior surface of the peritoneum; and Lobstein is of opinion that it is a continuation of the cellular tissue, which covers this membrane. (Lobstein, sur la Nutrition du Foetus. § 75.)

Externally, the umbilical cord is covered by a continuation of the amnion, which, although it be the inner membrane of the ovum, is the outer covering of the cord: in some places it is very thick and strong, and not easily ruptured. From repeated observations, the weakest part of the cord seems to be at about three or four inches distant from the umbilicus, this being the spot where it has invariably given way in every case we have seen, where the cord has been broken at the moment of the child’s birth.

From the time of the commencement to the full time of utero-gestation, the cord becomes gradually longer, so that it attains an average length of from eighteen to twenty inches; this, however, varies remarkably. We have known the cord exceed forty inches; and a case is described by Baudelocque, where it was actually fifty-seven inches long: on the other hand, it is sometimes not more than four or five inches in length.

It is remarkable that the cord, which at the end of pregnancy is usually of about the same length as the foetus, is relatively much longer during the sixth month; hence we may conclude, that in those cases where knots have been found upon the cord, the knot must have been formed at this period when the foetus was small enough to pass through a coil of it.

Neither blood-vessels nor lymphatics have as yet been found in the structure of the cord itself. A filament of nerve from the solar plexus has been occasionally seen passing through the umbilical ring, and extending to a distance down the cord.

The vesicula umbilicalis and allantois, being essentially connected with the earliest grades of foetal development, will be considered under that head.

Embryo. There is, perhaps, no department of physiology which has been so remarkably enriched by recent discoveries, as that which relates to the primitive development of the ovum and its embryo. The researches of Baer, Rathke, Purkinje, Valentin, &c. in Germany; of Dutrochet, Prevost, Dumas, and Coste, &c. in France; and of Owen, Sharpey, Allen Thomson, Jones, and Martin Barry in England, but more especially those of the celebrated Baer, have greatly advanced our knowledge of these subjects, and led us deeply into those mysterious processes of Nature which relate to our first origin and formation.

These researches have all tended to establish one great law, connected with the early development of the human embryo, and that of other mammiferous animals, viz, that it at first possesses a structure and arrangement analogous to that of animals in a much lower scale of formation: this observation also applies of course to the ovum itself, since a variety of changes take place in it after impregnation, before a trace of the embryo can be detected.

At the earliest periods, the human ovum bears a perfect analogy to the eggs of fishes, amphibia, and birds; and it is only by carefully examining the changes produced by impregnation in the ova of these lower classes of animals, that we have been enabled to discover them in the mammalia and human subject.

As the bird’s egg, from its size, best affords us the means of investigating these changes, and as in all essential respects they are the same in the human ovum, it will be necessary for us to lay before our readers a short account of its structure and contents, and also of the changes which they undergo, after impregnation. In doing this we shall merely confine ourselves to the description of what is applicable to the human ovum.

The egg is known to consist of two distinct parts, the vitellus or yelk surrounded by its albumen or white; to the former of these we now more particularly refer. The yelk is a granular albuminous fluid, contained in a granular membranous sac (the blastodermic membrane) which is covered by an investing membrane called the vitelline membrane or yelk-bag. The impregnated vitellus is retained in its capsule in the ovary, precisely as the ovum of the mammifera is in the Graafian vesicle. The whole ovary in this case has a clustered appearance, like a bunch of grapes, each capsule being suspended by a short pedicle of indusium.

In those ova which are considerably developed before impregnation, the granular blastermodic membrane is observed to be thicker, and the granules more aggregated at that part which corresponds to the pedicle, forming a slight elevation with a depression in its centre, like the cumulus in the proligerous disc of a Graafian vesicle. This little disc is the blastoderma, germinial membrane or cicatricula; in the central depression just mentioned is an exceedingly minute vesicle first noticed by Professor Purkinje of Breslau, and named after him: in more correct language it is the germinal vesicle.

According to Wagner, the germinal vesicle is not surrounded by a disc before impregnation; and it is only after this process that the above-mentioned disc of granules is formed. By the time the ovum is about to quit the ovary the vesicle itself has disappeared, so that an ovum has never been found in the oviduct containing a germinal vesicle, nothing remaining of it beyond the little depression in the cumulus of the cicatricula.

The rupture of the Purkinjean or germinal vesicle has been supposed by Mr. T. W. Jones to take place before impregnation; but the observations of Professor Valentin seem to lead to the inference that it is a result of that process, and must be therefore looked upon as one of the earliest changes which take place in the ovum or yelk-bag upon quitting the ovary.[24]

During its passing through the oviduct (what in mammalia is called the Fallopian tube,) the ovum receives a thick covering of albumen, and as it descends still farther along the canal the membrane of the shell is formed.On examining the appearance of the ovum in mammiferous animals, and especially the human ovum, it will be found that it presents a form and structure very analogous to the ova just described, more especially those of birds. It is a minute spherical sac, filled with an albuminous fluid, lined with its blastodermic or germinal membrane, in which is seated the germinal vesicle or vesicle of Purkinje. When the ovum has quitted the ovary the germinal vesicle disappears, and on its entering the Fallopian tube it becomes covered with a gelatinous, or rather albuminous covering. This was inferred by Valentin, who considered that “the enormous swelling of the ova, and their passage through the Fallopian tubes,” tended to prove the circumstance. (Edin. Med. and Surg. Journ. April, 1836.) It has since been demonstrated by Mr. T. W. Jones in a rabbit seven days after impregnation. The vitellary membrane seems, at this time, to give way, leaving the vitellus of the ovum merely covered by its spherical blastoderma, and encased by the layer of albuminous matter which surrounds it.

From what we have now stated, a close analogy will appear between the ova of the mammalia and those of the lower classes, more especially birds, which from their size afford us the best opportunities of investigating this difficult subject.

In birds, the covering of the vitellus is called yelk-bag; whereas, in mammalia and man it receives the name of vesicula umbilicalis. Its albuminous covering, which corresponds to the white and membrane of the shell in birds, is called chorion: by the time that the ovum has reached the uterus, this outer membrane has undergone a considerable change; it becomes covered with a complete down of little absorbing fibrillÆ, which rapidly increase in size as development advances, until it presents that tufted vascular appearance, which we have already mentioned when describing this membrane.

The first or primitive trace of the embryo is in the cicatricula or germinal membrane, which contained the germinal vesicle before its disappearance. In the centre of this, upon its upper surface, may be discovered a small dark line;[25] “this line or primitive trace is swollen at one extremity, and is placed in the direction of the transverse axis of the egg.”

a Transparent area. b Primitive trace.

As development advances, the cicatricula expands. “We are indebted to Pander,”[26] says Dr. Allen Thomson in his admirable essay above quoted, “for the important discovery, that towards the twelfth or fourteenth hour, in the hen’s egg the germinal membrane becomes divided into two layers of granules, the serous and mucous layers of the cicatricula; and that the rudimentary trace of the embryo, which has at this time become evident, is placed in the substance of the upper-most or serous layer.” “According to this observer, and according to Baer, the part of this layer which surrounds the primitive trace soon becomes thicker; and on examining this part with care, towards the eighteenth hour, we observe that a long furrow has been formed in it, in the bottom of which the primitive trace is situated; about the twentieth hour this furrow is converted into a canal open at both ends, by the junction of its margins (the plicÆ primitivÆ of Pander, the laminÆ dorsales of Baer:) the canal soon becomes closed at the cephalic or swollen extremity of the primitive trace, at which part it is of a pyriform shape, being wider here than at any other part. According to Baer and Serres, some time after the canal begins to close, a semi-fluid matter is deposited in it, which on its acquiring greater consistence, becomes the rudiment of the spinal cord; the pyriform extremity or head is soon after this seen to be partially subdivided into three vesicles, which being also filled with a semi-fluid matter, gives rise to the rudimentary state of the encephalon.” “As the formation of the spinal canal proceeds, the parts of the serous layer which surrounds it, especially towards the head, become thicker and more solid, and before the twenty-fourth hour we observe on each side of this canal four or five small round opaque bodies, these bodies indicate the first formation of the dorsal vertebrÆ.

a Transparent area. b LaminÆ dorsales. c Cephalic end. d Rudiments of dorsal vertebrÆ. e Serous layer. f Lateral portion of the primitive trace. g Mucous layer. h Vascular layer. k LaminÆ dorsales united to form the spinal canal.

“About the same time, or from the twentieth to the twenty-fourth hour, the inner layer of the germinal membrane undergoes a farther division, and by a peculiar change is converted into the vascular mucous layers.” (A. Thomson, op. cit.) It will thus be seen, that the germinal membrane is that part of the ovum in which the first changes produced by impregnation are observed. The rudiments of the osseous and nervous systems are formed by the outer or serous layers; the outer covering of the foetus or integuments, including the amnois, are also furnished by it. “The layer next in order has been called vascular, because in it the development of the principal parts of the vascular system appears to take place. The third, called the mucous layer, situated next the substance of the yelk, is generally in intimate connexion with the vascular layer, and it is to the changes which these combined layers undergo, that the intestinal, the respiratory, and probably also the glandular systems owe their origin.” (A. Thomson, op. cit. p. 298.)

a Serous layer. b c Vascular layer. d Mucous layer. e Heart.

The embryo is therefore formed in the layers of the germinal membrane, and becomes, as it were, spread out upon the surface of the ovum: the changes which the ovum of mammalia undergoes appear from actual observation, to be precisely analogous to those in the inferior animals. (Baer, Prevost and Dumas.) From the primitive trace, which was at first merely a line crossing the cicatricula, and which now begins rapidly to exhibit the characters of the spinal column, the parietes of the head and trunk gradually approach farther and farther towards the anterior surface of the abdomen and head until they unite; in this way the sides of the jaws close in the median line of the face, occasionally leaving the union incomplete, and thus appearing to produce in some cases the congenital defects of hare-lip and cleft palate. In some way the ribs meet at the sternum; and it may be supposed that sometimes this bone is left deficient, and thus may become one of the causes of those rare cases of malformation, where the child has been born with the heart external to the parietes of the thorax. In like manner the parietes of the abdomen and pelvis close in the linea alba and symphysis pubis, occasionally leaving the integuments of the navel deficient, or, in other words, producing congenital umbilical hernia, or at the pubes a non-union of its symphysis with a species of inversion of the bladder, the anterior wall of that viscus being nearly or entirely wanting.

The cavity of the abdomen is therefore at first open to the vesicula umbilicalis or yelk, but this changes as the abdominal parietes begin to close in; in man and the mammalia merely a part of it, as above mentioned, forms the intestinal canal, whereas, in oviparous animals the whole of the yelk-bag enters the abdominal cavity, and serves for an early nutriment to the young animal. Another change connected with the serous or outer layer of the germinal membrane is the formation of the amnion. The foetal rudiment which from its shape has been called carina, now begins to be enveloped by a membrane of exceeding tenuity, forming a double covering upon it; the one which immediately invests the foetus is considered to form the future epidermis; the other, or outer fold, forms a loose sac around it, containing the liquor amnii. Whilst these changes are taking place in the serous layer of the germinal membrane, and whilst the intestinal canal, &c. are forming on the anterior surface of the embryo, which is turned towards the ovum, by means of the inner or mucous layer, equally important changes are now observed in the middle or vascular layer. “In forming this fold,” says Dr. A. Thomson, “the mucous layer is reflected farthest inwards; the serous layer advances least, and the space between them, occupied by the vascular layer, is filled up by a dilated part of this layer, the rudiment of the heart.” (Op. cit. p. 301.)

Whilst this rudimentary trace of the vascular system is making its appearance, minute vessels are seen ramifying over the vesicula umbilicalis, forming, according to Baer’s observations, a reticular anastomosis, which unites into two vessels the vasa omphalo-meseraica. (British and Foreign Med. Rev. No. 1.) These may be demonstrated with great ease in the chick: the cicatricula increases in extent; it becomes vascular, and at length forms a heart-shaped net-work of delicate vessels, which unite into two trunks, terminating one on each side of the abdomen.

The umbilical vesicle now begins to separate itself more and more from the abdomen of the foetus, merely a duct of communication passing to that portion of it which forms the intestinal canal. The first rudiment of the cord will be found at this separation; its foetal extremity remains for a long time funnel-shaped, containing, besides a portion of intestine, the duct of the vesicula umbilicalis, the vasa omphalo-meseraica (the future vena portÆ,) the umbilical vein from the collected venous radicles of the chorion, and the early trace of the umbilical arteries. These last-named vessels ramify on a delicate membranous sac of an elongated form which rises from the inferior or caudal extremity of the embryo, viz. the allantois; whether this is formed by a portion of the mucous layer of the germinal vesicle, in common with the other abdominal viscera, appears to be still uncertain: in birds this may be very easily demonstrated as a vascular vesicle, arising from the extremity of the intestinal canal; and in mammalia, connected with the bladder by means of a canal called urachus: from its sausage-like shape, it has received the name of allantois.

The existence of an allantois in the human embryo has been long inferred from the presence of a ligamentous cord extending from the fundus of the bladder to the umbilicus, like the urachus in animals. But from the extreme delicacy of the allantois, and from its function ceasing at a very early period, it had defied all research, until lately when it has been satisfactorily demonstrated in the human embryo by Baer and Rathke. It occupies the space between the chorion and amnion, and gives rise occasionally to a collection of fluid between these membranes, familiarly known by the name of the liquor amnii spurius, which, strictly speaking is the liquor allantoidis.

The function of the allantois is still in a great measure unknown. In animals it evidently acts as a species of receptaculum urinÆ during the latter periods of gestation; but it is very doubtful if this be its use during the earlier periods. It does not seem directly connected with the process of nutrition, which at this time is proceeding so rapidly, first by means of the albuminous contents of the vitellus, or vesicula umbilicalis, and afterwards by the absorbing radicles of the chorion; but, from analogy with the structure of the lower classes of animals, it would appear that it is intended to produce certain changes in the rudimentary circulation of the embryo, similar to those which, at a later period of pregnancy, are effected by means of the placenta, and after birth by the lungs, constituting the great functions of respiration.

In many of the lower classes of animals, respiration (or at least the functions analogous to it) is performed by organs situated at the inferior or caudal extremity of the animal: thus for instance, certain insect tribes, as in hymenoptera, or insects with a sting, as wasps, bees, &c.; in diptera, or insects with two wings, as the common fly; and also the spider tribe, have their respiratory organs situated in the lower part of the abdomen. In some of the crustacea, as, for instance, the shrimp, the organs of respiration lie under the tail between the fins, and floating loosely in the water. Again, some of the molusca, viz. the cuttle-fish, have the respiratory organs in the abdomen. We also know that many animals, during the first periods of their lives, respire by a different set of organs to what they do in the adult state: the most familiar illustration of this is the frog, which, during its tadpole state, lives entirely in the water.

As the growth of the embryo advances, other organs whose function is as temporary as that of the allantois, make their appearance: these also correspond to the respiratory organs of a lower class of animals, although higher than those to which we have just alluded,—we mean bronchial processes or gills. It is to Professor Rathke (Acta NaturÆ Curios. vol. xiv,) that we are indebted for pointing out the interesting fact, that several transverse slit-like apertures may be detected on each side the neck of the embryo, at a very early stage of development. In the chick, in which he first observed it, it takes place about the fourth day of incubation: at this period the neck is remarkably thick, and contains a cavity which communicates inferiorly with the oesophagus and stomach, and opens externally on each side by means of the above-mentioned apertures, precisely as is observed in fishes, more especially the shark tribe; these apertures are separated from each other by lobular septa, of exceedingly soft and delicate structure. Rathke observed the same structure in the embryo of the pig and other mammalia; and Baer has since shown it distinctly in the human embryo. It is curious to see how the vascular system corresponds to the grade of development then present: the heart is single, consisting of one auricle and one ventricle; the aorta gives off four delicate, but perfectly simple branches, two of which go to the right, and two to the left side; each of these little arteries passes to one of the lobules or septa at the side of the neck, which correspond to gills, and having again united with the three others, close to what is the first rudiment of the vertebral column, they form a single trunk which afterwards becomes the abdominal aorta. In a short time these slit-like openings begin to close; the bronchial processes or septa become obliterated, and indistinguishable from the adjacent parts; the heart loses the form of a single heart; a crescentic fold begins to mark the future division into two ventricles, and gradually extends until the septum between them is completed. It is also continued along the bulb of the aorta, dividing it into two trunks, the aorta proper and pulmonary artery; at the upper part the division is left incomplete, so that there is an opening from one vessel to the other, which forms the ductus arteriosus.[27] A similar process takes place in the auricles, the foramen ovale being apparently formed in the same manner as the ductus arteriosus; these changes commence in the human embryo about the fourth week, and are completed about the seventh.

At first the body of the embryo has a more elongated form than afterwards, and the part which is first developed is the trunk, at the upper extremity of which a small prominence less thick than the middle part, and separated from the rest of the body by an indentation, distinguishes the head. There are as yet no traces whatever of extremities, or of any other prominent parts; it is straight, or nearly so, the posterior surface slightly convex, the anterior slightly concave, and rests with its inferior extremity directly upon the membranes, or by means of an extremely short umbilical cord.

The head now increases considerably in proportion to the rest of the body, so much so, that at the beginning of the second month, it equals nearly half the size of the whole body: previous to, and after this period, it is usually smaller. The body of the embryo becomes considerably curved, both at its upper as well as its lower extremity, although the trunk itself still continues straight. The head joins the body at a right angle, so that the part of it which corresponds to the chin is fixed directly upon the upper part of the breast; nor can any traces of neck be discerned, until nearly the end of the second month.

The inferior extremity of the vertical column, which at first resembles the rudiment of a tail becomes shorter towards the middle of the third month, and takes a curviture forwards under the rectum, in the fifth week the extremities become visible, the upper usually somewhat sooner than the lower, in the form of small blunt prominences. The upper close under the head, the lower near the caudal extremity of the vertebral column. Both are turned somewhat outwards, on account of the size of the abdomen; the upper are usually directed somewhat downwards, the lower ones somewhat upwards.

The vesicula umbilicalis may still be distinguished in the second month as a small vesicle, not larger than a pea, near the insertion of the cord, at the navel, and external to the amnion. From the trunk, which is almost entirely occupied by the abdominal cavity, arises a short thick umbilical cord, in which some of the convolutions of the intestines may still be traced. Besides these it usually contains, as already observed, the two umbilical arteries and the umbilical vein, the urachus, the vasa omphalo-meseraica, or vein and artery of the vesicula umbilicalis, and perhaps, even at this period, the duct of communication between the intestinal canal and vesicula umbilicalis, the foetal extremity of which, according to Professor Oken’s views, forms the processus vermiformis.

Diagram of the foetus and membranes about the sixth week.

a Chorion. b The larger absorbent extremities, the site of the placenta. c Allantois. d Amnion. e Urachus. É Bladder. f Vesicula umbilicalis. g Communicating canal between the vesicula umbilicalis and intestine. h Vena umbilicalis. i i ArteriÆ umbilicales. l Vena omphalo-meseraica. k Arteria omphalo-meseraica. n Heart. o Rudiment of superior extremity. p Rudiment of lower extremity. From Carus.

The hands seem to be fixed to the shoulders without arms, and the feet to adhere to the ossa illi; the liver seems to fill the whole abdomen; the ossa innominata, the ribs, and scapulÆ are cartilaginous.

In a short time the little stump-like prominences of the extremities become longer, and are now divided into two parts, the superior into the hand and the fore arm, the inferior into the foot and leg; in one or two weeks later, the arms and thighs are visible. These parts of the extremities which are formed later than the others, are at first smaller, but as they are gradually developed they become larger. When the limbs begin to separate into an upper and lower part, their extremities become rounder and broader, and divided into the fingers and toes, which at first are disproportionately thick, and until the end of the third month are connected by a membranous substance analogous to the webbed feet of water birds; this membrane gradually disappears, beginning at the extremities of the fingers and toes, and continuing the division up to their insertion. The external parts of generation, the nose, ears, and mouth appear after the development of the extremities. The insertion of the umbilical cord changes its situation to a certain degree; instead of being nearly at the inferior extremity of the foetus as at first, it is now situated higher up on the anterior surface of the abdomen. The comparative distance between the umbilicus and pubis continues to increase, not only to the full period of gestation, when it occupies the middle point of the length of the child’s body, as pointed out by Chaussier, but even to the age of puberty, from the relative size of the liver becoming smaller.

Though the head appears large at first, and for a long time continues so, yet its contents are tardy in their development, and until the sixth month the parietes of the skull are in great measure membranous or cartilaginous. Ossification commences in the base of the cranium, and the bones under the scalp are those in which this process is last completed.

The contents of the scull are at first gelatinous, and no distinct traces of the natural structure of the brain can be identified until the close of the second month; even then it requires to have been sometimes previously immersed in alcohol to harden its texture. There are many parts of it not properly developed until the seventh month. In the medulla spinalis no fibres can be distinguished until the fourth month. The thalami nervorum opticorum, the corpora striata, and tubercula quadrigemina, are seen in the second month; in the third, the lateral and longitudinal sinuses can be traced, and contain blood. In the fifth we can distinguish the corpus callosum; but the cerebral mass has yet acquired very little solidity, for until the sixth month it is almost semi-fluid. (Campbell’s System of Midwifery.)

About the end of the third, during the fourth, and the beginning of the fifth months, the mother begins to be sensible of the movements of the foetus. These motions are felt sooner or later, according to the bulk of the child, the size and shape of the pelvis, and the quantity of fluid contained in the amnion, the waters being in larger proportionate quantity the younger the foetus.

The secretion of bile, like that of the fat, seems to begin towards the middle of pregnancy, and tinges the meconium, a mucous secretion of the intestinal tube which had hitherto been colourless, of a yellow colour. Shortly after this the hair begins to grow, and the nails are formed about the sixth or seventh month. A very delicate membrane (membrana pupillaris,) by which the pupil has been hitherto closed, now ruptures, and the pupil becomes visible. The kidneys, which at first were composed of numerous glandular lobules (seventeen or eighteen in number,) now unite, and form a separate viscus on each side of the spine; sometimes they unite into one large mass, an intermediate portion extending across the spine, forming the horse-shoe kidney.

Lastly, the testes, which at first were placed on each of the lumbar vertebrÆ, near the origin of the spermatic vessels, now descend along the iliac vessels towards the inguinal rings, directed by a cellular cord, which Hunter has called Gubernaculum testis: they then pass through the openings carrying before them that portion of the peritoneum which is to form their tunica vaginalis.

The length of a full-grown foetus is generally about eighteen or nineteen inches; its weight between six and seven pounds. The different parts are well developed and rounded; the body is generally covered with the vernix caseosa;[28] the nails are horny, and project beyond the tips of the fingers, which is not the case with the toes; the head has attained its proper size and hardness; the ears have the firmness of cartilage; the scrotum is rugous, not peculiarly red, and usually containing the testes. In female children the nymphÆ are generally covered entirely by the labia, the breasts project, and in both sexes frequently contain a milky fluid. As soon as a child is born, which has been carried the full time, it usually cries loudly, opens its eyes, and moves its arms and legs briskly; it soon passes urine and fÆces, and greedily takes the nipple. (NaegelÉ’s Hebammenbuch.)

Thus, then, in the space of forty weeks, or ten lunar months, from an inappreciable point, the foetus attains a medium length of about eighteen or nineteen inches, and a medium weight of between six and seven pounds. As these observations on the development of the ovum show that the structural arrangement of the embryo undergoes a succession of changes, by which it gradually rises from the lowest to the highest scale of formation, so we shall find it furnished with a succession of means for its nutrition, each corresponding more or less to the particular grade of development which it may have attained. Its earliest source of nourishment is doubtless the vitellus, or albuminous contents of the vesicula umbilicalis. The radicle or primitive trace, in this respect, bears a strong analogy to the seed of a plant; it brings with it its own supply of nourishment for its first stage of growth; in the latter, the cotyledons afford nourishment to the little plumula, until, by the formation of roots and absorption of moisture from the surrounding soil, it is enabled to support the early rudiment of the future plant. The early function of the chorion is very analogous to that of roots; it is an absorbing apparatus, collecting nourishment by means of its numerous absorbing fibrillÆ: hence, according to Lobstein, the umbilical vein exists for some time previous to the umbilical arteries, and seems to perform an office in the foetus similar to that of the thoracic duct at a later period; its radicles or absorbing extremities seem to absorb a milky fluid, which after the first two months is found in the placenta, and which must be looked upon as a means of nourishment which does not exist in the latter months. This milky fluid was noticed by Leroux, who even then expressed his doubts, whether the radicles of the umbilical vein receive blood from the mother, or whether they only serve to absorb a white fluid which resembles chyle. In some manuscript notes of Dr. Young’s lectures, which were taken by the late Dr. Parry, of Bath, when a student at Edinburgh, we find the following observation: “There is evidently in the placenta, besides blood-vessels, some other substance, which serves to absorb juices from the uterus, and to convert these into a chylous matter proper to nourish the foetus, and this matter is absorbed by the umbilical veins. This seems to be proved from the consideration of the placenta of animals which have cotyledons; for, on squeezing these glandular substances, we force out a sort of chylous liquor, and these are surrounded by the placenta, which absorb their liquor and convey it to the foetus.”

The absorbing power of the umbilical vein continues till the fifth month; during the second or third, the foetus receives a good deal of nourishment from the liquor amnii, which at this period contains a considerable quantity of albuminous matter; this diminishes in the latter months of pregnancy. Moreover the body of the foetus begins to be covered with the vernix caseosa towards the seventh month, so that in the eighth and ninth months the absorption of liquor amnii by the skin is considerably impeded.

How far the full formed placenta, as seen after the fifth month, serves as a means of nutrition to the foetus, may still be a matter of doubt; its chief use after this period is, as we have already shown, for the purpose of producing certain changes in the blood of the foetus analogous to those of respiration;[29] still, however, it would seem that its function of nutrition is not entirely at an end, even at a late period of pregnancy. The numerous little granules of phosphate of lime, which are frequently found on the uterine surface of a full-grown placenta at a time when ossification is rapidly advancing in the foetal skeleton, would surely lead us to infer that the placenta in some way or other supplies the materials for this process.

Foetal circulation. We have already shown, that, in the early stages of development, the heart of the embryo is single, consisting of one auricle and one ventricle; that a septum gradually divides these into two parts until the double heart is formed, leaving two openings of communication between the right and left sides, the one between the auricles called the foramen ovale, the other between the pulmonary artery and aorta, viz. the ductus arteriosus.

From these and other peculiarities it will be seen that the foetal circulation differs essentially from that of a child after birth; and, in order to comprehend the nature and mechanism of the changes which take place in it when respiration first commences, it will be necessary that these peculiarities should be thoroughly understood. The condition of the foetus must also be remembered: surrounded by the liquor amnii, the foetus does not respire; its lungs have as yet been unemployed; they are therefore small and collapsed, and present a firm solid mass, nearly resembling liver in appearance. In this state but little blood from the pulmonary arteries can circulate through them; for, as the extreme ramifications of these vessels are distributed upon the mucous membrane lining the bronchi and air-cells, the free passage of blood through them will in great measure depend upon a previous condition of the air-cells. The pulmonary arteries in the foetal state are therefore small, and transmit but a small quantity of blood into their numerous ramifications, just sufficient to keep pervious these vessels which after birth are to be so greatly distended: in this state the lungs when thrown into water sink.

Hence, as the pulmonary arteries do not afford a sufficiently free exit to the contents of the right side of the foetal heart, nature has provided it with a peculiar means for carrying off the overplus quantity of blood, which is poured into the right auricle from the vena cava. This is attained first by the foramen ovale, an oval-shaped opening in the septum between the right and left auricles, and furnished with a semilunar valvular flap, so constructed, as to allow a free passage for the blood from the right to the left auricle, but none in the contrary direction. By this means a considerable quantity of blood is transmitted at once from the right to the left auricle, and, consequently, much less into the right ventricle and pulmonary artery. Still, however, more blood passes into the right ventricle than the pulmonary artery, in the collapsed state of the foetal lungs, is capable of conveying away. The pulmonary artery is therefore continued beyond its bifurcation into the aorta at its curvature, by means of the ductus arteriosus, which, in the full-grown foetus, forms a short thick passage between these two vessels; and in this manner is the right ventricle enabled to get rid of its surplus quantity of blood. Thus we see that the foetal heart although consisting of two auricles and two ventricles, continues to perform the functions only of a single heart, both ventricles assisting simultaneously to propel the same column of blood, viz. that of the aorta, and thus enabling the heart to act with considerable power.

The chief part of the blood, which flows through the iliac arteries, instead of being sent to the inferior extremities, is carried into the umbilical arteries, which passing up along the sides of the bladder meet the umbilical vein at the navel, and thus form the vessels of the umbilical cord. These arteries convey the blood of the foetus to the placenta, where, having undergone changes to which we have already alluded, it is returned by the umbilical vein. This vessel, which afterwards forms the round ligament of the liver, passes through the umbilicus along the anterior edge of the suspensory ligament; it supplies the left lobe with blood, and having given off a communicating branch to the vena portÆ, which supplies the right lobe, it passes at once by a short passage, called canalis venosus, into the vena cava.

Thus, then, the peculiarities of the foetal circulation may be considered as four, viz. the foramen ovale, or passage from the right to the left auricle; the ductus arteriosus, or communication from the bifurcation of the pulmonary artery into the arch of the aorta; the umbilical arteries arising from the iliac arteries, and carrying the blood along the cord into the placenta; and, lastly, the canalis venosus, or passage between the umbilical vein and vena cava.

Let us now examine the changes which take place in the foetal circulation at the moment of the child’s birth. The child, which had hitherto been immersed in the bland and warm medium of the liquor amnii, is at once exposed to the action of the external air. By means of the sympathy existing between the skin and respiratory muscles, sudden and convulsive efforts at inspiration take place; the air-cells of the lungs become partially inflated, and, after a short time as the respiration increases in power and activity, become distended throughout their whole extent. The thorax rises; the flaccid diaphragm, which hitherto had been pushed up by the large foetal liver, now contracts, pressing down the liver into its natural situation. The lungs, from being a hard solid heavy substance, resembling liver, at once become inflated, elastic, and crepitous, light and permeable to air in every part.

The capillary terminations of the pulmonary artery, which ramify in the mucous membrane, forming the parietes of the air-cells, and which hitherto had been firmly compressed by the collapsed state of the foetal lungs, are suddenly rendered pervious throughout their whole extent. By this means, a vacuum, as it were, is formed in the ramifications of the pulmonary artery; each inspiration is accompanied by a rush of blood from the right ventricle into the newly-inflated structure. The pulmonary artery, at its bifurcation, swells and becomes turgid: the blood is carried off into its numerous ramifications as fast as the right ventricle can supply it; this may be easily understood from the law, in anatomy, viz. that the area of two arteries is greater than that of the trunk from which they bifurcate. From this state of distension, the distance between the pulmonary artery and the aorta is increased; the ductus arteriosus, which has now become empty, is stretched, and thus partially closed; the right auricle, which, but for the foramen ovale, could not have cleared itself of the whole quantity of blood which was poured into it from the vena cava, is now enabled to transmit its entire contents into the right ventricle; the left auricle, which before birth was supplied only by the foramen ovale from the right auricle, is now rapidly filled by the blood brought into it by the four pulmonary veins;—the equilibrium between the two auricles becomes altered;—the right, which hitherto had been somewhat gorged with blood, is now able to clear itself with facility; whereas, the left, which was but partially supplied, is now distended with a much greater quantity: there is now rather a disposition for the blood to regurgitate from the left to the right auricle; this, however, is prevented by the semilunar fold of the foramen ovale, which now acts as a valve, and generally becomes firmly attached to the septum. The obliteration of the canalis venosus at the posterior margin of the liver, and of the umbilical vein at the anterior edge, may, we think, be explained by the changes which necessarily follow the inflation of the lungs: the diaphragm, when it contracts, pulls down the liver into its natural situation; the distance, therefore, between the liver and the heart is increased, and the canalis venosus is consequently stretched, and considerably pressed upon, and precisely the same results follow with the umbilical vein.