Being Well-Born: An Introduction to Eugenics

CHAPTER II

THE BEARERS OF THE HERITAGE

Before we can make any detailed analysis of the inheritance of characters we should have some general idea of the physical structure of animals and particularly some familiarity with the development of an individual from the egg, as well as some knowledge of the nature of the germ-cells.

The Cell the Unit of Structure.—If we examine one of the higher animals, as, for example, the horse, the dog, or man, we find that it is made up of a large number of constituents, such as bones, muscles, nervous elements, blood and other tissues. Each kind of tissue is composed of a number of living units, ordinarily microscopic in size, which are known as cells. A careful examination of various cells reveals that although they may differ greatly in size, shape and minor details, they all alike possess certain well-marked characteristics. Each when reduced to its fundamental form is seen to consist of a small mass of living matter termed protoplasm in which may usually be distinguished two regions—the cell-body or cytoplasm, and the nucleus (Fig. 3, p. 21). Any cell, whether it be of the brain, of the liver, or from any organ of an animal or plant, has this same fundamental structure. In addition, a limiting membrane or wall of some kind is generally present, although it is not a necessary constituent of all cells.

Fig. 3

Diagram of a cell showing various parts.

Unicellular Organisms.—While such a structure as a tree or a horse is composed of countless millions of cells, on the other hand numerous organisms, both plant and animal, exist which consist of only one cell. Yet this cell is just as characteristically a cell as are the components of a complex animal or plant. It has the necessary parts, the cell body and the nucleus. Moreover it exhibits all of the fundamental activities of life, though in a simplified form, that a complex higher organism does.

Importance of Cell-Theory.—This discovery that every living thing is a single cell or an aggregation of cooperating cells and cell-products is one of our most important biological generalizations because it has brought such a wide range of phenomena under a common point of view. In the first place, the structure of both plants and animals is reducible to a common fundamental unit of organization. Moreover, both physiological and pathological phenomena are more readily understood since we recognize that the functions of the body in health or disease are in large measure the result of the activities of the individual cells of the functioning part. Then again, the problems of embryological development have become much more sharply defined since it could be shown that the egg is a single cell and that it is through a series of divisions of this cell and subsequent changes in the new cells thus formed that the new organism is built up. And lastly, the problem of hereditary transmission has been rendered more definite and approachable by the discovery that the male germ is likewise a single cell, that fertilization of the egg is therefore the union of two cells, and that in consequence the mechanism of inheritance must be stowed away somehow in these two cells.

Heredity in Unicellular Forms.—In unicellular animals one can readily see how it is possible for an individual always to give rise to its own kind. One of the simplest of the single-celled animals is the Ameba (Fig. 4, p. 24).

The ameba eats and grows as do other animals. Sooner or later it reaches a size beyond which it can not increase advantageously, yet it is continuously taking in new food material which stimulates it to further growth. Here then is a problem. The ameba solves this difficulty by dividing to form two amebÆ. Such a division is illustrated in Fig. 4, p.24. First the nucleus divides, then the cell-body. When the two new amebÆ separate completely each renews the occupation of eating and growing. But what has become of the parent? Here, where once existed a large adult ameba are two young amebÆ. The parent individual as such has disappeared, yet there has been no death, for we have simply two bits of living jelly in place of one. They will in turn repeat the same process, so will their offspring, and thus, barring accident, this growth and reproduction, or overgrowth as we may regard it, may go on forever, as far as we know. Here the problem of heredity, or the resemblance of offspring to parent, is not a very complicated one. The substance of the cell-body and cell-nucleus divides into two similar halves, so that each descendant has the substance of the parent in its own body, only it has but half as much. It differs from the parent, not in quality or kind, but in size.

Fig. 4

Six successive stages in the division of Ameba polypodia (after Schulze). The nucleus is seen as a dark spot in the interior.

Reproduction and Heredity in Colonial Protozoa.—There are enormous numbers of these single-celled animals existing in all parts of the world. Some are simple like the ameba, others are very complex in structure. Many, after division, move apart and pursue wholly independent courses of existence. On the other hand we find a modification appearing in some which is of the greatest importance. After division instead of moving apart the two cells may remain side by side and divide further to form two more, these in turn may divide and thus the process goes on until there is formed what is known as a colony. Each cell of such a colony resembles the original ancestral cell because each is a part of the actual substance of that cell. As in the ameba, the first two cells are the ancestral cell done up in two separate packets, and thus finally the full quota of cells must be so many separate packets of the same kind of material. Inasmuch as each is but a repetition of its original ancestor, it can, and at times does, produce a colony of the same kind as that ancestor produced.

Conjugation.—At longer or shorter intervals, however, we find that two individuals, on the disruption of the old colony, instead of continuing the routine of establishing new colonies through a series of cell divisions, very radically alter their behavior. They unite and fuse into a single larger individual. This process is called conjugation. We find it occurring even in some species of ameba. The conjugating cells in some colonies are alike in size and appearance, in others different.

Specialization of Sex-Cells.—A beautiful sphere-shaped colony known as Volvox is to be found occasionally in roadside pools. Depending on the species of Volvox to which it belongs, the colony may be made up of from a few hundred to several thousand individuals arranged in a single layer about the fluid-filled center of the sphere and bound together by a clear jelly-like inter-cellular substance. Each individual cell also connects with its neighbors by means of thin threads of living matter. One of the largest species is Volvox globator, one edge of which is represented in Fig. 5, p. 27. Mutual pressure of the cells gives them a polygonal shape when viewed from the surface. Each cell, with a few exceptions to be noted immediately, bears two long flagella, whip-like structures which project out into the water. The lashing of these flagella gives the ball a rotary motion and thus it moves about. When the colony has reached its adult condition and is ready to reproduce itself, certain cells without flagella and somewhat larger than the ordinary cells become more rounded in outline and increase considerably in size through the acquisition of food materials. They are then known as egg cells or ova. Each ovum finally enters on a series of cell-divisions forming a mass of smaller and smaller cells which gradually assumes the form of a hollow sphere like the parent colony. The young colonies thus formed drop into the interior of the parent colony to escape later to the outside as independent swimming organisms when the old colony dies and disintegrates.

The Fertilized Ovum Termed a Zygote.—After a number of generations of such asexual reproduction, sexual reproduction occurs. The ova arise as usual. Certain members of the colony, on the other hand, go to the other extreme and divide up into bundles of from sixty-four to one hundred twenty-eight minute slender cells, each provided with flagella for locomotion. When mature these small flagellate cells, now known as spermatozoa, escape into the interior of the parent colony and swim about actively. Ultimately each ovum is penetrated by a spermatozoon, the two cells fuse completely and thus form the single fertilized ovum or zygote. The body-cells of the mother colony finally disintegrate. After a period of rest each zygote, through a series of cell-divisions, develops into an adult Volvox. In some species of Volvox a still further advance is seen, in that instead of both kinds of gametes being produced in the same colony, the ova may be produced by one colony and the spermatozoa by another. Here, then, we have the foreshadowings of two sexes as separate individuals, a phenomenon of universal occurrence among the highest forms of animal life.

Fig. 5

Volvox globator (from Hegner after Oltmanns). Half of a sexually reproducing colony: o, eggs; s, spermatozoa.

Advancement Seen in the Volvox Colony.—In the Volvox colony there is a distinct advance over the conditions met with in various lower protozoan colonies in that only certain individuals of the colony take part in the process of reproduction and these individuals are of two distinct types; one is a larger, food-laden cell or egg and the other a small, active, fertilizing cell. The motile forms are produced in much greater numbers than the eggs, plainly because they have to seek the egg and many will doubtless perish before this can be accomplished. This disparity in number is only a means of insuring fertilization of the egg. The remaining cells of the body carry on the ordinary activities of the colony such as locomotion and nutrition and have ceased to take any part in the production of new colonies.

Natural Death Appears With the Establishment of a Body Distinct from the Germ.—Volvox is an organism of unusual interest because in it we see a prophecy of what is to come. Although still regarded as a colony of single-celled individuals, it represents in reality a transition between the whole group of unicellular animals termed protozoa and the many celled animals characterized by the possession of distinct tissues, known as Metazoa. Moreover, it shows an interesting stage in the establishment of a body or soma distinct from special reproductive cells which have taken on the function of reproducing the colony. In such colonial forms natural death is found appearing for the first time, the reproductive cells alone continuing to perpetuate the species. Then again Volvox represents an important step in the establishment of sex in the animal kingdom for in its sexual reproduction the conjugating cells known as gametes are no longer alike in appearance but have become differentiated into definite ova and spermatozoa.In Volvox as in the other organisms which we have studied we find that all of the cells including the germ-cells are produced by the repeated division of a parent cell, and consequently each must contain the characteristic living substance of that parent. Many other forms might be cited to illustrate reproduction in single-celled animals, whether free or in colonies, but all such cases would be practically but repetitions or modifications of those we have already examined.

Specialization in Higher Organisms.—If we pass on to the higher animals and plants which are not single cells or colonies of similar cells but organisms made up of many different kinds of cells, we find a pronounced extension of the phenomenon met with in Volvox. Instead of each cell executing independently all of the life relations, certain ones are set apart for the performance of certain functions to the exclusion of other functions which are carried on by other members of the aggregation. Thus the organism as a whole has all the life relations carried on, but, as it were, by specialists.

Sexual Phenomena in Higher Forms.—In the reproduction of multicellular organisms, one sees likewise but a continuation of the phenomena exhibited in Volvox. Ordinarily, each new form is produced by the successive divisions of a single germ-cell which in the vast majority of cases has conjugated with another germ-cell. In the development of the egg, as the divisions proceed, groups of cells become modified for their particular work until the entire organism is completed. During development certain cells are set apart for reproduction of the form just as they were in Volvox. These two kinds of reproductive cells in multicellular organisms are derived ordinarily from two separate individuals known as male and female, though there are some exceptions. The main difference between these cells which will have to unite to form a single fertile germ-cell, is that they have specialized in different directions; one is small and active, the other large, food-laden and passive. But with two such germ-cells coming as they do from two individuals, one the male, the other the female, it is obvious that the actual living substance of which each germ is composed will be distinctive of its own parental line and that when the germs unite these distinctive factors commingle, hence the complications of double ancestry arise.

Structure of the Cell.—Before we can understand certain necessary details of the physical mechanism of inheritance we must inquire a little further into the finer structure of the cell and into the nature of cell division. A typical cell, as it would appear after treatment with various stains which bring out the different parts more distinctly, is shown in Fig. 3, p. 21. Typical, not that any particular kind of living cell resembles it very closely in appearance, but because it shows in a diagrammatic way the essential parts of a cell. In the diagram, there are two well-marked regions; a central nucleus and a peripheral cell-body or cytoplasm. Other structures are pictured but only a few of them need command our attention at present. At one side of the nucleus one observes a small dot or granule surrounded by a denser area of cytoplasm. This body is called the centrosome. The nucleus in this instance is bounded by a well-marked nuclear membrane and within it are several substances. What appear to be threads of a faintly staining material, the linin, traverse it in every direction and form an apparent network. The parts on which we wish particularly to rivet our attention are the densely stained substances scattered along or embedded in the strands of this network in irregular granules and patches. This substance is called chromatin. It takes its name from the fact that it shows great affinity for certain stains and becomes intensely colored by them. This deeply colored portion of the cell, the chromatin, is by most biologists regarded as of great importance from the standpoint of heredity. One or more larger masses of chromatin or chromatin-like material, known as chromatin nucleoli, are often present, and not infrequently a small spheroidal body, differing in its staining reactions from the chromatin-nucleolus and sometimes called the true nucleolus, exists.

Cell-Division.—In the simplest type of cell-division the nucleus first constricts in the middle, and finally the two halves separate. This separation is followed by a similar constriction and final division of the entire cell-body, which results in the production of two new cells. This form of cell-division is known as simple or direct division. Such a simple division, while found in higher animals, is less frequent and apparently much less significant than another type of division which involves profound changes and rearrangements of the nuclear contents. The latter is termed mitotic or indirect cell-division. Fig. 6, p. 33, illustrates some of the stages which are passed through in indirect cell-division. The centrosome which lies passively at the side of the nucleus in the typical cell (Fig. 6a, p. 33) awakens to activity, divides and the two components come to lie at the ends of a fibrous spindle. In the meantime, the interior of the nucleus is undergoing a transformation. The granules and patches of chromatin begin to flow together along the nuclear network and become more and more crowded until they take on the appearance of one or more long deeply-stained threads wound back and forth in a loose skein in the nucleus (Fig. 6b, p. 33). If we examine this thread closely, in some forms it may be seen to consist of a series of deeply-stained chromatin granules packed closely together intermingled with the substance of the original nuclear network.

As the preparations for division go on the coil in the nucleus breaks up into a number of segments which are designated as chromosomes (Fig. 6c, p. 33). The nuclear membrane disappears. The chromosomes and the spindle-fibers ultimately become related in such a way that the chromosomes come to lie at the equator of the spindle as shown in Fig 6d, p. 33. Each chromosome splits lengthwise to form two daughter chromosomes which then diverge to pass to the poles of the spindle (Figs. 6e and f, p. 33). Thus each end of the spindle comes ultimately to be occupied by a set of chromosomes. Moreover each set is a duplicate of the other, because the substance of any individual chromosome in one group has its counterpart in the other. In fact this whole complicated system of indirect division is regarded by most biologists as a mechanism for bringing about the precise halving of the chromosomes.

Fig. 6

Diagram showing representative stages in mitotic or indirect cell-division: a, resting cell with reticular nucleus and single centrosome; b, the two new centrosomes formed by division of the old one are separating and the nucleus is in the spireme stage; c, the nuclear wall has disappeared, the spireme has broken up into six separate chromosomes, and the spindle is forming between the two centrosomes; d, equatorial plate stage in which the chromosomes occupy the equator of the spindle; e, f, each chromosome splits lengthwise and the daughter chromosomes thus formed approach their respective poles; g, reconstruction of the new nuclei and division of the cell body; h, cell-division completed.

The chromosomes of each group at the poles finally fuse and two new nuclei, each similar to the original one, are constructed (Figs. 6g and h, p. 33). In the meantime a division of the cell-body is in progress which, when completed, results in the formation of two complete new cells.

As all living matter if given suitable food, can convert it into living matter of its own kind, there is no difficulty in conceiving how the new cell or the chromatin material finally attains to the same bulk that was characteristic of the parent cell. In the case of the chromatin, indeed, it seems that there is at times a precocious doubling of the ordinary amount of material before the actual division occurs.

Chromosomes Constant in Number and Appearance.—With some minor exceptions, to be noted later, which increase rather than detract from the significance of the facts, the chromosomes are always the same in number and appearance in all individuals of a given species of plants or animals. That is, every species has a fixed number which regularly recurs in all of its cell-divisions. Thus the ordinary cells of the rat, when preparing to divide, each display sixteen chromosomes, the frog or the mouse, twenty-four, the lily twenty-four, and the maw-worm of the horse only four. The chromosomes of different kinds of animals or plants may differ very much in appearance. In some they are spherical, in others rod-like, filamentous or perhaps of other forms. In some organisms the chromosomes of the same nucleus may differ from one another in size, shape and proportions, but if such differences appear at one division they appear at others, thus showing that in such cases the differences are constant from one generation to the next.

Significance of the Chromosomes.—The question naturally arises as to what is the significance of the chromosomes. Why is the accurate adjustment which we have noted for their division necessary? The very existence of an elaborate mechanism so admirably adapted to their precise halving, predisposes one toward the belief that the chromosomes have an important function which necessitates the retention of their individuality and their equal division. Many biologists accept this along with other evidence as indicating that in chromatin we have a substance which is not the same throughout, that different regions of the same chromosome have different physiological values.

When the cell prepares for divisions, the granules, as we have seen, arrange themselves serially into a definite number of strands which we have termed chromosomes. Judging from all available evidence, the granules are self-propagating units; that is, they can grow and reproduce themselves. So that what really happens in mitosis in the splitting of the chromosomes is a precise halving of the series of individual granules of which each chromosome is constituted, or in other words each granule has reproduced itself. Thus each of the two daughter cells presumably gets a sample of every kind of chromosomal particle, hence, the two cells are qualitatively alike. To use a homely illustration we may picture the individual chromosomes to ourselves as so many separate trains of freight cars, each car of which is loaded with different merchandise. Now, if every one of the trains could split along its entire length and the resulting halves each grow into a train similar to the original, so that instead of one there would exist two identical trains, we should have a phenomenon analogous to that of a dividing chromosome.

Cleavage of the Egg.—It is through a series of such divisions as these that the zygote or fertilized egg-cell builds up the tissues and organs of the new organism. The process is technically spoken of as cleavage. Cleavage generally begins very shortly after fertilization. The fertile egg-cell divides into two, the resulting cells divide again and thus the process continues, with an ever-increasing number of cells.

Chief Processes Operative in Building the Body.—Although of much interest, space will not permit of a discussion in detail of the building up of the special organs and tissues of the body. It must suffice merely to mention the four chief processes which are operative. These are, (1) infoldings and outfoldings of the various cell complexes; (2) multiplication of the component cells; (3) special changes (histological differentiation) in groups of cells; and (4) occasionally resorption of certain areas of parts.

The Origin of the New Germ-Cells.—On account of the unusual importance from the standpoint of inheritance, which attaches to the germ-cells, a final word must be said about their origin in the embryo. While the evidence is conflicting in some cases, in others it has been well established that the germ-cells are set apart very early from the cells which are to differentiate into the ordinary body tissues. Fig. 7A, p. 38, shows a section through the eight-celled stage of Miastor, a fly, in which a single large, primordial germ-cell (p. g. c.) has already been set apart at one end of the developing embryo. The nuclei of the rest of the embryo still lie in a continuous protoplasmic mass which has not yet divided up into separate cells. The densely stained nuclei at the opposite end of the section are the remnants of nurse-cells which originally nourished the egg. Fig. 7B, p. 38, is a longitudinal section through a later stage in the development of Miastor; the primitive germ-cells (oÖg) are plainly visible. Still other striking examples might be cited. Even in vertebrates the germ-cells may often be detected at a very early period.

Significance of the Early Setting Apart of the Germ-Cells.—It is of great importance for the reader to grasp the significance of this early setting apart of the germ-cells because so much in our future discussion hinges on this fact. The truth of the statement made in a previous chapter that the body of an individual and the reproductive substance in that body are not identical now becomes obvious. For in such cases as those just cited one sees the germinal substance which is to carry on the race set aside at an early period in a given individual; it takes no part in the formation of that individual’s body, but remains a slumbering mass of potentialities which must bide its time to awaken into expression in a subsequent generation. Thus an egg does not develop into a body which in turn makes new germ-cells, but body and germ-cells are established at the same time, the body harboring and nourishing the germ-cells, but not generating them (Fig. 2, p. 13). The same must be true also in many cases where the earliest history of the germ-cells can not be visibly followed, because in any event, in all higher animals, they appear long before the embryo is mature and must therefore be descendants of the original egg-cell and not of the functioning tissues of the mature individual. This need not necessarily mean that the germ-cells have remained wholly unmodified or that they continue uninfluenced by the conditions which prevail in the body, especially in the nutritive blood and lymph stream, although as a matter of fact most biologists are extremely skeptical as to the probability that influences from the body beyond such general indefinite effects as might result from under-nutrition or from poisons carried in the blood, modify the intrinsic nature of the germinal substances to any measurable extent.

Fig. 7

A—Germ-cell (p. g. c.) set apart in the eight-celled stage of cleavage in Miastor americana (after Hegner). The walls of the remaining seven somatic cells have not yet formed though the resting or the dividing (M p) nuclei may be seen; c R, chromatin fragments cast off from the somatic cells.

B—Section lengthwise of a later embryo of Miastor; the primordial egg-cells (oÖg₃) are conspicuous (after Hegner).

Germinal Continuity.—The germ-cells are collectively termed the germinal protoplasm and it is obvious that as long as any race continues to exist, although successive individuals die, some germinal protoplasm is handed on from generation to generation without interruption. This is known as the theory of germinal continuity. When the organism is ready to reproduce its kind the germ-cells awaken to activity, usually undergoing a period of multiplication to form more germ-cells before finally passing through a process of what is known at maturation, which makes them ready for fertilization. The maturation process proper, which consists typically of two rapidly succeeding divisions, is preceded by a marked growth in size of the individual cells.

Individuality of Chromosomes.—Before we can understand fully the significance of the changes which go on during maturation we shall have to know more about the conditions which prevail among the chromosomes of cells. As already noted each kind of animal or plant has its own characteristic number and types of chromosomes when these appear for division by mitosis. In many organisms the chromosomes are so nearly of one size as to make it difficult or impossible to be sure of the identity of each individual chromosome, but on the other hand, there are some organisms known in which the chromosomes of a single nucleus are not of the same size and form (Fig. 8, p. 41). These latter cases enable us to determine some very significant facts. Where such differences of shape and proportion occur they are constant in each succeeding division so that similar chromosomes may be identified each time. Moreover, in all ordinary mitotic divisions where the conditions are accurately known, these chromosomes of different types are found to be present as pairs of similar elements; that is, there are two of each form or size.

Pairs of Similar Chromosomes in the Nucleus Because One Chromosome Comes from Each Parent.—When we recall that the original fertilized egg from which the individual develops is really formed by the union of two gametes, ovum and spermatozoon, and that each gamete, being a true cell, must carry its own set of chromosomes, the significance of the pairs of similar chromosomes becomes evident; one of each kind has probably been contributed by each gamete. This means that the zygote or fertile ovum contains double the number of chromosomes possessed by either gamete, and that, moreover, each tissue-cell of the new individual will contain this dual number. For, as we have seen, the number of chromosomes is, with possibly a few exceptions, constant in the tissue-cells and early germ-cells in successive generations of individuals. For this to be true it is obvious that in some way the nuclei of the conjugating gametes have come to contain only half the usual number. Technically the tissue-cells are said to contain the diploid number of chromosomes, the gametes the reduced or haploid number.

Fig. 8

A—Chromosomes of the mosquito (Culex) after Stevens.

B—Chromosomes of the fruit-fly (Drosophila) after Metz.

Both of these forms have an unusually small number of chromosomes.

In Maturation the Number of Chromosomes Is Reduced by One-Half.—This halving, or as it is known, reduction in the number of chromosomes is the essential feature of the process of maturation. It is accomplished by a modification in the mitotic division in which instead of each chromosome splitting lengthwise, as in ordinary mitosis, the chromosomes unite in pairs (Fig. 9b, p. 42), a process known technically as synapsis, and then apparently one member of each pair passes entire into one new daughter cell, the other member going to the other daughter cell (Fig. 9c, p. 42). In the pairing preliminary to this reduction division, leaving out of account certain special cases to be considered later, according to the best evidence at our command the union always takes place between two chromosomes which match each other in size and appearance. Since one of these is believed to be of maternal and the other of paternal origin, the ensuing division separates corresponding mates and insures that each gamete gets one of each kind of chromosome although it appears to be a matter of mere chance whether or not a given cell gets the paternal or the maternal representative of that kind.

Fig. 9

Diagram to illustrate spermatogenesis: a, showing the diploid number of chromosomes (six is arbitrarily chosen) as they occur in divisions of ordinary cells and spermatogonia; b, the pairing (synapsis) of corresponding mates in the primary spermatocyte preparatory to reduction; c, each secondary spermatocyte receives three, the haploid number of chromosomes; d, division of the secondary spermatocytes to form e, spermatids, which transform into f, spermatozoa.

Maturation of the Sperm-Cell.—In the maturation of the male gamete the germ-cell, now known as a spermatogonium, increases greatly in size to become a primary spermatocyte. In each primary spermatocyte the pairing of the chromosomes already alluded to occurs as indicated in Fig. 9b, p. 42, where six is taken arbitrarily to indicate the ordinary or diploid number of chromosomes, and three the reduced or haploid number. The division of the primary spermatocyte gives rise to two secondary spermatocytes (c), the paired chromosomes separating in such a way that a member of each pair goes to each secondary spermatocyte. Each secondary spermatocyte (d) soon divides again into two spermatids (e), but in this second division the chromosomes each split lengthwise as in an ordinary division so that there is no further reduction. In some forms the reduction division occurs in the secondary spermatocytes instead of the primary. Each spermatid transforms into a mature spermatozoon (f). The spermatozoa of most animals are of linear form, each with a head, a middle-piece and a long vibratile tail which is used for locomotion. The head consists for the most part of the transformed nucleus and is consequently the part which bears the chromosomes.Maturation of the Egg-Cell.—As regards the behavior of the chromosomes the maturation of the ovum parallels that of the sperm-cell. There are not so many primordial germ-cells formed and only one out of four of the ultimate cells becomes a functional egg. As in maturation of the sperm-cell there is a growth period in which oÖgonia enlarge to become primary oÖcytes (Fig. 10b, p. 45). In each primary oÖcyte as in the primary spermatocyte the chromosomes pair and two rapidly succeeding divisions follow in one of which the typical numerical reduction in the chromosomes occurs. A peculiarity in the maturation of the ovum is that there is a very unequal division of the cytoplasm in cell division so that three of the resulting cells usually termed polar bodies are very small and appear like minute buds on the side of the fourth or egg-cell proper.

The scheme of this formation of the polar bodies is indicated in Fig. 10, p. 45. In Fig. 10b the chromosomes are seen paired and ready for the first division; that is, for the formation of the first polar body. Figs. 10c, d, p. 45, show the giving off of this body. Note that while only a small proportion of the cytoplasm passes into this tiny cell, its chromatin content is as great as that of the ovum. A second polar body (Figs. 10e, f, p. 45) is formed by the egg, but in this case each chromosome splits lengthwise, as in ordinary mitosis, and there is no further numerical reduction. In the meantime, typically, a third polar body is formed by division of the first. (Stages e, f, g.)

Parallel Between the Maturation of Sperm- and Egg-Cell.—This rather complex procedure of the germ-cells will be rendered more intelligible through a careful study of Figs. 9 and 10, pp. 42 and 45, and Fig. 11, p. 46, which indicates the parallel conditions in spermotogenesis and oÖgenesis.

Fig. 10

Diagram to illustrate oÖgenesis: a, showing the diploid number of chromosomes (six is arbitrarily chosen) as they occur in ordinary cells and oÖgonia; b, the pairing of corresponding mates preparatory to reduction; c, d, reduction division, giving off of first polar body; e, egg preparing to give off second polar body, first polar body ready for division; f, g, second polar body given off, division of first polar body completed. The egg nucleus, now known as the female pronucleus, and each body contain the reduced or haploid number of chromosomes.

The view now generally held regarding the polar bodies is that they are really abortive eggs. They later disappear, taking no part in embryo-formation. It can readily be seen how such an unequal division is advantageous to the large cell, for it receives all of the rich store of food material that would be distributed among the four cells if all were of equal size. This increased amount of food is a favorable provision for the forthcoming offspring whose nourishment is thus more thoroughly insured.

Larger Image

Fig. 11

Diagram showing the parallel between maturation of the sperm-cell and maturation of the ovum.

On the other hand, all of the sperm-cells develop into complete active forms, which, as aforesaid, usually become very much elongated and develop a motile organ of some kind. In such cells an accumulation of food to any large extent would hinder rather than help them, because it would seriously interfere with their activity.

Fertilization.—In fertilization (Fig. 12, p. 48) the spermatozoon penetrates the wall of the ovum and after undergoing considerable alteration its nucleus fuses with the nucleus of the egg. In some forms only the head (nucleus) and middle-piece enter, the tail being cut off by a so-called fertilization membrane which forms at the surface of the egg and effectually blocks the entrance of other spermatozoa. Thus normally only one spermatozoon unites with an egg. In some forms while several may enter the egg only one becomes functional. As soon as the nucleus of the spermatozoon, now known as the male pronucleus, reaches the interior of the egg, it enlarges and becomes similar in appearance to the female pronucleus. It swings around in such a way (Fig. 12b, p. 48) that the middle piece, now transformed into a centrosome, lies between it and the female pronucleus. The two pronuclei (c, d, e), each containing the reduced number of chromosomes, approach, the centrosome divides, the nuclear walls disappear, the typical division spindle forms, and the chromosomes of paternal and maternal origin respectively come to lie side by side at the equator of the spindle ready for the first division or cleavage (f, g). It will be noted that the individual chromosomes do not intermingle their substance at this time, but that each apparently retains its own individuality. There is considerable evidence which indicates that throughout life the chromosomes contributed by the male parent remain distinct from those of the female parent. Inasmuch as each germ-cell, after maturation, contains only half the characteristic number of chromosomes, the original number is restored in fertilization.

Fig. 12

Diagram to illustrate fertilization; ?, male pronucleus; ?, female pronucleus; observe that the chromosomes of maternal and paternal origin respectively do not fuse.

Significance of the Behavior of the Chromosomes.—The question confronts us as to what is the significance of this elaborate system which keeps the chromosomes of constant size, shape and number; which partitions them so accurately in ordinary cell-divisions; and which provides for a reduction of their numbers by half in the germ-cell while yet securing that each mature gamete gets one of each kind of chromosome. Most biologists look on these facts as indicating that the chromosomes are specifically concerned in inheritance.

In the first place it is recognized that as regards the definable characters which separate individuals of the same species, offspring may inherit equally from either parent. And it is a very significant fact that while the ovum and spermatozoon are very unequal in size themselves, the chromosomes of the two germ-cells are of the same size and number. This parity in chromosomal contribution points clearly to the means by which an equal number of character-determiners might be conveyed from each parent. Moreover it is mainly the nucleus of the sperm-cell in some organisms which enters the egg, hence the determiners from the male line must exist wholly or largely somewhere in the nucleus. And the bulk of the nucleus in the spermatozoon consists of the chromosomes or their products.

A Single Set of Chromosomes Derived from One Parent Only Is Sufficient for the Production of a Complete Organism.—That a single or haploid set of chromosomes as seen in the gametes is sufficient contribution of chromatin for the production of a complete organism is proved by the fact that the unfertilized eggs of various animals (many echinoderms, worms, mollusks, and even the frog) may be artificially stimulated to development without uniting at all with a spermatozoon. The resulting individual is normal in every respect except that instead of the usual diploid number it has only the single or haploid number of chromosomes. Its inheritance of course is wholly of maternal origin. The converse experiment in echinoderms in which a nucleus of male origin (that is, a spermatozoon) has been introduced into an egg from which the original nucleus has been removed shows that the single set of chromosomes carried by the male gamete is also sufficient to cooperate with the egg-cytoplasm in developing a complete individual.

The Duality of the Body and the Singleness of the Germ.—Since every maternal chromosome in the ordinary cell has an equivalent mate derived from the male parent, it follows therefore, supposing the chromosomes do have the significance in inheritance attributed to them, that as regards the measurable inheritable differences between two individuals, the ordinary organism produced through the union of the two germ-cells is, potentially at least, dual in nature. On the other hand through the process of reduction the gametes are provided with only a single set of such representatives. This duality of the body and singleness of the mature germ is one of the most striking facts that come to light in embryology. How well the facts fit in with the behavior of certain hereditary characters will be seen later in our discussions of Mendelism.

The Cytoplasm Not Negligible in Inheritance.—Just what part is played by the cytoplasm in inheritance is not clear, but it is probably by no means a negligible one. The cytoplasm of a given organism is just as distinctive of the species or of the individual of which it forms a part as are the chromosomes. It is well established that neither nucleus nor cytoplasm can fully function or even exist long without the other, and neither can alone produce the other. They undoubtedly must cooperate in building up the new individual, and the cytoplasm of the new individual is predominantly of maternal origin. It is obvious that all of the more fundamental characters which make up an organism, such, for instance, as make it an animal of a certain order or family, as a human being or a dog or a horse, are common to both parents, and there is no way of measuring how much of this fundamental constitution comes from either parent, since only closely related forms will interbreed. In some forms, moreover, the broader fundamental features of embryogeny are already established before the entrance of the spermatozoon. It is probable therefore that instead of asserting that the entire quota of characters which go to make up a complete individual are inherited from each parent equally, we are justified only in maintaining that this equality is restricted to those measurable differences which veneer or top off, as it were, the individual. We may infer that in the development of the new being the chromosomes of the egg together with those derived from the male work jointly on or with the other germinal contents which are mostly cytoplasmic materials of maternal origin.

The Chromosomes Possibly Responsible for the Distinctiveness of Given Characters.—It seems probable that in the establishment of certain basic features of the organism the cooperation of the cytoplasm with chromatin of either maternal or paternal origin might accomplish the same end, but that certain distinctive touches are added or come cumulatively into expression through influences carried, predominantly at least, in the chromatin from one as against the other parent. These last distinctive characters of the plant or animal constitute the individual differences of such organisms. In this connection it is a significant fact that in young hybrids between two distinct species the early stages of development, especially as regards symmetry and regional specifications, are exclusively or predominantly maternal in character, but the male influence becomes more and more apparent as development progresses until the final degree of intermediacy is attained.

From the evidence at hand this much seems sure, that the paternal and maternal chromosomes respectively carry substances, be they ferments, nutritive materials or what not, that are instrumental in giving the final parity of personal characters which we observe to be equally heritable from either line of ancestry. It is clear that most of the characters of an adult organism can not be merely the outcome of any unitary substance of the germ. Each is the product of many cooperating factors and for the final outcome any one cooperant is probably just as important in its way as any other. The individual characters which we juggle to and fro in our breeding experiments seem apexed, as it were, on more fundamental features of organic chemical constitution, polarity, regional differentiation, and physiological balance, but since such individual characters parallel so closely the visible segregations and associations which go on among the chromosomes of the germ-cells it would seem that they, at least, are represented in the chromosomes by distinctive cooperants which give the final touch of specificity to those hereditary characters which can be shifted about as units of inheritance.

Sex and Heredity.—Whatever the origin of fertilization may have been in the world of life, or whatever its earliest significance, the important fact remains that to-day it is unquestionably of very great significance in relation to the phenomena of heredity. For in all higher animals, at least, offspring may possess some of the characteristics originally present in either of two lines of ancestry, and this commingling of such possessions is possible only through sexual reproduction. As has already been seen, in the pairing of chromosomes previous to reduction, the corresponding members of a pair always come together so that in the final segregation each gamete is sure to have one of each kind although whether a given chromosome of the haploid set is of maternal or paternal origin seems to be merely a matter of chance. Thus, for instance, if we arbitrarily represent the chromosomes of a given individual by ABC abc, and regard A, B and C as of paternal and a, b and c as of maternal origin, then in synapsis only A and a can pair together, B and b and C and c, but each pair operates independently of the other so that in the ensuing reduction division either member of a pair may get into a cell with either member of the other pairs. That is, the line up for division at a given reduction might be any one of the following, ^ABC/abc ^ABc/abC ^Abc/aBC ^AbC/aBc. This would yield the following eight kinds of gametes, ABC, abc, ABc, abC, Abc, aBC, AbC, aBc, each bearing one of each kind of chromosome required to cover the entire field of characters necessary to a complete organism. And since each sex would be equally likely to have these eight types of gametes and any one of the eight in one individual might meet any one of the eight of the other, the possible number of combinations in the production of a new individual from such germ-cells would be 8x8, or 64. With the larger numbers of chromosomes which exist in most animals it is readily seen that the number of possible combinations becomes very great. Thus any individual of a species with twenty chromosomes—and many animals, including man, have more—would have ten pairs at the reduction period and could therefore form (2)¹⁰, or 1,024 different gametes in each sex. And since any one of these in one sex would have an equal chance of meeting with any one in the opposite sex, the total number of possible different zygotes that might be produced would be (1,024)², or 1,048,576. Sex therefore, through recombinations of ancestral materials, undoubtedly means, among other things, the production of great diversity in offspring.

DETERMINATION OF SEX

Many Theories.—From earliest times the problem of sex determination has been one of keen interest, and needless to say hundreds of theories have been propounded to explain it. Geddes and Thomson say that Drelincourt recorded two hundred sixty-two so-called theories of sex production and remark that since his time the number has at least been doubled. The desirability of controlling sex has naturally appealed strongly to breeders of domesticated animals.

A study of animals born in litters, or of twins, is enough in itself to make us skeptical of theories of sex-determination based on nutritional or external factors. In a litter of puppies, for example, there are usually both males and females, although in their prenatal existence they have all been subject to the same nutritional and environmental conditions. Likewise in ordinary human twins one may be a boy, the other a girl, whereas if the nutritional condition of the mother were the fact determining sex, both should be boys or both girls. However, there are twins known as identical twins who are remarkably alike and who are always of the same sex. But there is reason to suppose that identical twins in reality come from the same zygote. Presumably in early embryogeny, probably at the two-celled stage of cleavage, the two blastomeres become separated and each gives rise to a complete individual instead of only the half of one it would have produced had the two blastomeres remained together. Such twins are monochorial; that is, they grow inside the same fetal membrane, whereas each ordinary twin has its own fetal membrane and has obviously originated from a separate ovum. It has been established experimentally in several kinds of animals that early cleavage blastomeres when isolated can each develop into a complete individual. In man, ordinary twins are no more alike than ordinary brothers and sisters, but identical twins are strikingly similar in structure, appearance, habits, tastes, and even susceptibility to various maladies. The fact that they are invariably of the same sex is a strong reason for believing that sex was already developed in the fertile ovum and consequently in the resulting blastomeres from that ovum.

The young of the nine-banded armadillo in a given litter are invariably of the same sex and are closely similar in all features. Newman and Patterson have shown that all the members of a litter come from the same egg. Patterson has established the fact that cleavage of the egg takes place in the usual manner, but later separate centers of development appear in the early embryonic mass and give rise to the separate young individuals.

Again in certain insects where one egg indirectly gives rise to a chain of embryos, or to a number of separate larvÆ, possibly as many as a thousand, all of the latter are of the same sex. Even in some plants researches have shown that sex is already determined at the beginning of development. Then, too, much evidence has come to light recently showing that sex-characters in certain cases behave as heritable characters and are independent of external conditions. Lastly there is visible and convincing evidence obtainable through microscopical observations that sex is determined by a mechanism in the germ-cells themselves. It is chiefly to these latter facts that I wish to direct attention for the present.

The Sex Chromosome.—The evidence centers about a special chromosome or chromosome-group commonly designated as the sex-chromosome or X-element, which has been found in various species of animals, including man. In the males of such animals this chromosome is present in addition to the regular number of pairs, thus giving rise to an uneven instead of the conventional even number of chromosomes. This element remains undivided in one of the maturation divisions of the spermatocytes, in some forms in the first in others in the second, and passes entire to one pole of the spindle (Fig. 13, p. 58). This results in the production of two classes of cells, one containing the X-element and one not. The outcome is that two corresponding classes of spermatozoa are produced. The phenomena involved are diagrammatically represented in Fig. 13. It has been clearly demonstrated in several cases that eggs fertilized by spermatozoa which possess this X-element, always become females, those fertilized by spermatozoa which do not possess it always develop into males.

Fig. 13

Diagram illustrating the behavior of the x-element or sex-chromosome in the maturation of the sperm-cell. In one of the two maturation divisions (represented here as in the first) it passes undivided to one pole (a, b, c), in the other it divides. Since the cell without the x-element also divides the result is that ultimately from the original primary spermatocyte (a) four cells are formed (f), two with the x-element and two without it. Half of the spermatozoa therefore will bear an x-element, half will be without it. In a the ordinary chromosomes, arbitrarily indicated as 10, are supposed to have already paired for reduction so that the original diploid number in spermatogonia and body-cells of the male would be 20 plus the x-chromosome.

It has been found, furthermore, that in species in which the males possess this extra element the females have two of them. That is, if the original number in the somatic cells of the male were twenty-three, twenty-two ordinary and one X-element, the number in the somatic cells of the female would be twenty-four, or twenty-two ordinary and two X-elements. It has been found that when the chromosomes of the female pair for the reduction division, each chromosome uniting with its corresponding fellow, the two X-elements in the female pair in the usual way so that every egg-cell possesses an X-element. Thus every mature egg has an X-element, while only half of the spermatozoa have one. That is, if we assume twenty-three as the diploid number present originally in the somatic cells of the male and twenty-four as the number in the female, then one-half the spermatozoa of the male would contain the haploid number eleven, and the other half, the number twelve, whereas every mature ovum would contain twelve. Since there are equal numbers of the spermatozoa with the X-element and without it, and inasmuch as presumably under ordinary conditions one kind is as likely to fertilize the egg as the other, then there are equal chances at fertilization of producing a zygote with two X-elements or with but one.

Thus,	Spermatozoon + X by Ovum + X = Zygote + XX.
	Spermatozoon (no X) by Ovum + X = Zygote + X.

We have already seen that the former is always female, the latter male. It thus becomes possible to distinguish the sex of an embryo by counting the chromosomes of its cells. This has been accomplished in several cases.

In some instances[1] the conditions may be much more complex than the ones indicated—too complex in fact to warrant detailed discussion in an elementary exposition—but the principle remains the same throughout, the very complexity when thoroughly understood, strengthening rather than weakening the evidence. In a few forms an interesting reversal of conditions has been found in that the eggs instead of the spermatozoa show the characteristic dimorphism.

Just what the exact relationship between sex-differentiation and the X-element is has never been clearly established. It is possible that this element is an actual sex-determinant, in the ordinary cases one X-element determining the male condition and two X-elements producing the female condition. On the other hand it might be argued that it is not the determining factor but the expression of other cell activities which do determine sex; that is, a sex accompaniment. Or again, it may be one of several essential factors which must cooperate to determine sex.

SEX-LINKED CHARACTERS

The discovery of the remarkable behavior of certain characters in heredity which can only be plausibly explained by supposing that they are linked with a sex-determining factor still further strengthens our belief in the existence of such a definite factor. Such characters are commonly termed sex-linked characters.

Sex-Linked Characters in Man.—Since there are a number of them in man we may choose one of these, such as color-blindness, for illustration. The common form of color-blindness known as Daltonism in which the subject can not distinguish reds from greens, a condition which seems to be due to the absence of something which is present in individuals of normal color vision, is far commoner in men than in women. Its type of inheritance, sometimes termed “crisscross” heredity, has been likened to the knight moves in a game of chess. The condition is transmitted from a color-blind man through his daughter to half of her sons. Or, to go more into detail, a color-blind father and normal mother have only normal children whether sons or daughters. The sons continue to have normal children but the daughters, although of normal vision themselves, transmit color-blindness to one-half of their own sons. If such a woman marries a color-blind man, as might easily happen in a marriage between cousins, then as a rule one-half her daughters as well as one-half her sons will be color-blind.

Fig. 14

Diagram illustrating the inheritance of a sex-limited character such as color-blindness in man on the assumption that the factor in question is located in the sex-chromosome (from Loeb after Wilson). The normal sex-chromosome is indicated by a black X, the one lacking the factor for color perception, by a light X. It is assumed that a normal female is mated with a color-blind male.

In such cases what appears to be a mysterious procedure becomes very simple if we assume that the defective character is associated with the sex-determining factor, or to make it concrete let us say with the X-element. The chart shown in Fig. 14, p. 62, indicates what the germinal condition would be under the circumstances. The column to the right represents the maternal, the one to the left the paternal line. Since two X means female and one X male, and inasmuch as we have assumed that the physical basis of the defect to which color-blindness is due is conveyed by the X-element, we may represent the defective single X of the male in outline only (see first row). It is obvious that after the reduction divisions (second row) the mature sex-cells of the female will each contain a single normal X, the corresponding sex cells of the male will contain either no X or a defective X. Since if any member of the class of spermatozoa containing no X, fertilizes an egg the resulting zygote (row three) will have but one X and that a normal one, the individual which develops from the zygote will be normal as regards color vision and moreover will be male because the condition one X always means maleness. On the other hand, if any member of the class of spermatozoa containing the defective X fertilizes an egg two X-elements are brought together and this of itself means femaleness. In this case one of the X-elements is defective but the single normal X is sufficient in itself to produce normal color vision. But when it comes to the maturation of the sex-cells of this female, the pair of X-elements are separated in the usual way with the result that half of the mature ova contain a normal X and half a defective X (row four). Since in a normal male, however, the mature reproductive cells will contain either a normal X or no X (fourth row), any one of four different kinds of matings may result. A sex-cell carrying normal X of the male may combine with an ovum containing normal X producing a normal female (row five). Or such a cell may combine with an ovum carrying the defective X, also producing a female but one who although of normal color vision herself, like her mother, is a carrier of the defect. On the other hand, any one of the spermatozoa without an X may combine with an ovum containing the normal X, in which case a normal male is produced and, moreover, one who, like his mother’s brothers, is incapable of transmitting the defect. However, the sperm-cell devoid of an X is just as likely to fertilize an ovum carrying the defective X, in which event the resulting individual, a male, must be color-blind because he contains the defective X alone. In other words, the chances are that one-half the sons of a woman whose father was color-blind will be color-blind, the other half perfectly normal; and that all of the daughters will be of normal color vision although one-half of them will probably transmit the defect to one-half of their sons. From a glance at the diagram it is readily seen also that a color-blind female could result from the union of a color-blind man (see first row) and the daughter of a color-blind man (see third row). For half of the gametes of such a female would bear the defect as would also that half of the gametes of the male which carry X, hence the expectation would be that half of the daughters of such a union would be color-blind and half would be carriers of color-blindness; and that half of the sons would be color-blind and half normal. All the sons of a color-blind woman would be color-blind because she has only defective X-elements to pass on.

The inheritance of various other conditions in man follows more or less accurately the same course as color-blindness. Among these may be mentioned: hemophilia, a serious condition in which the blood will not clot properly, thus rendering the affected individual constantly liable to severe or fatal hemorrhage; near-sightedness (myopia) in some cases; a degenerative disease of the spinal cord known as multiple sclerosis; progressive atrophy of the optic nerve (neuritis optica); Gower’s muscular atrophy; some forms of night-blindness; in some cases ichthyosis, a peculiar scaly condition of the skin. In one of my own tabulations of a case of inheritance of “webbed” digits or syndactyly, a condition in which two or more fingers or toes are more or less united, a sex-linked inheritance is clearly indicated (Fig. 15), although from the pedigrees recorded by other investigators this condition usually appears in some of both the sons and daughters of an affected individual.

Fig. 15

Chart showing the inheritance of a case of syndactyly after the manner of a sex-linked character. The affected individuals are represented in black; squares indicate males, circles females. The condition is seen to be inherited by males through unaffected females.

The Occurrence of Sex-Linkage in Lower Forms Renders Experiments Possible.—The course followed by such characters in man can be inferred only from the pedigrees we can obtain from family histories. Fortunately, however, such sex-linkage also occurs in lower animals and we are able therefore to verify and extend our observations by direct experiments in breeding. Several sex-linked characters have been found to exist in a small fruit-fly known as Drosophila. Extensive breeding experiments with this fly by Professor T. H. Morgan and his pupils have borne out remarkably the interpretation that the characters in question are really linked with a sex-determining factor.

Clyx.com