During the normal life of an individual many of the tissues of the body are being continuously renewed, or replaced at definite periods. The replacement of a part may go on by a process of continuous growth, such as takes place in the skin and nails of man, or the replacement may be abrupt, as when the feathers of a bird are moulted. It is the latter kind of process that is generally spoken of as physiological regeneration. In the same animal, however, certain organs may be continually worn away, and as slowly replaced, and other organs replaced only at regular intervals.

Bizozzero has made the following classification of the tissues of man, on the basis of their power of physiological regeneration. (1) Tissues made up of cells that multiply throughout life, as the parenchyma cells of those glands that form secretions of a definite morphological nature; the tissues of the testes, marrow; lymph glands, ovaries; the epithelium of certain tubular glands of the digestive tract and of the uterus; and the wax glands. (2) Tissues that increase in the number of their cells till birth, and only for a short time afterward, as the parenchyma of glands with fluid secretions, the tissues of the liver, kidney, pancreas, thyroid, connective tissue, and cartilage. (3) Tissues in which multiplication of cells takes place only at an early embryonic stage, as striated muscles and nerve tissues. In these there is no physiological regeneration.

There are many familiar cases of periodic loss of parts of the body. The hair of some mammals is shed in winter and in summer. Birds renew their feathers, as a rule, once a year. Snakes shed their skin from time to time. The antlers of deer are thrown off each year, and new ones formed accompanied by an increase in size and branching of the antlers. In other cases similar changes may be associated with certain stages in the life of the animal. The milk-teeth of the mammals are lost at definite periods, and new teeth acquired.[61] The larval exoskeleton of insects is thrown off at intervals, and after each moult the body increases in size; but after the pupa stage is passed and the imago formed, there is no further moulting. In the crustacea, on the other hand, the adult animals moult from time to time, and the upper limit of size is less well defined than in the insects. The larvÆ also pass through a series of moults.

An interesting case of physiological regeneration has been described by Balbiani in a unicellular form, stentor. From time to time a new peristome appears along the side, moves forward and replaces the old peristome, that is absorbed as the new one comes into position. In other infusoria the peristome may be absorbed before encystment, and a new one appears when the animal emerges from the cyst. Schuberg states that when division takes place in bursaria the new peristome develops on the aboral piece in the same way as after encystment; and Gruber observed that, when an aboral piece of an infusorian is cut off, a new peristome develops in the same way as after normal division of the animal. These observations indicate that the process of physiological regeneration may follow the same course and probably involves the same factors as the process of restorative regeneration.

Tubularia absorbs its old hydranth-heads if placed in an aquarium, and regenerates new ones. It may even absorb the hydranth while growing in an aquarium, as Dalyell has shown, and presumably, therefore, also under natural conditions. After each regeneration the new stalk behind the head increases in length.

In plants, in which there is a continuous apical growth, new parts are being always added at the end of the stem, and old parts are continually dying, as seen in palms. Most trees and shrubs in temperate climates lose their leaves once a year and produce new ones in the spring. Since the new leaves develop from the new shoots at the end of the stem and branches, the old ones can, only in a general way, be said to be renewed.

That a very close relation exists between the process of physiological regeneration and restorative regeneration will be sufficiently evident from the preceding illustrations. We do not gain any insight into either of the processes, so far as I can see, by deriving the one from the other, for the process of restorative regeneration may be, in point of time, as old as that of physiological regeneration. This does not mean, of course, that the same factors may not be present in both cases. So similar are the two processes that several naturalists have attempted to show how the process of restorative regeneration has been derived from physiological regeneration. Barfurth, recognizing the resemblance between the two processes, speaks of restorative regeneration as a modification of physiological regeneration, and Weismann also supports this point of view. He says: “Physiological and pathological regeneration obviously depend on the same causes, and often pass one into the other, so that no real line of demarcation can be drawn between them. We nevertheless find that in those animals in which the power of regeneration is extremely great physiologically, it is very slight pathologically. This proves that a slight power of pathological regeneration cannot possibly depend on a general regenerative force present within the organism, but rather that this power can be provided in those parts of the body which require a continual, periodic regeneration; in other words, the regenerative power of a part depends on adaptation.” It is, I think, erroneous to state “that in those animals in which the power of regeneration is extremely great physiologically, it is very slight pathologically.” All that we are justified in concluding from the evidence is that in some cases in which physiological regeneration takes place, as in the vertebrates, pathological (restorative) regeneration may not be well developed; but even in these forms restorative regeneration is certainly present, and present especially in internal organs, as in the salivary gland, in the liver, and in the eye, which are little exposed to injury. How far physiological regeneration takes place in the tissues of the lower animals we do not know at present, except in a few cases, but far from supposing it to be absent, it may be as well developed as in higher forms. Weismann’s further conclusion, that because in some animals physiological regeneration is very great and restorative regeneration very slight, therefore the latter cannot “depend on a general regenerative force within the organism,” is, I think, quite beside the mark. In this connection we should not fail to notice a difference between these two regenerative processes that several writers have also called attention to, viz. that the power of cell-multiplication and the formation of new cells in each kind of tissue does not carry with it the power of restorative or even of physiological regeneration, in cases where several kinds of tissue make up an organ. For instance, if the leg of the mammal is cut off, the old cells may give rise to new ones, but the processes that would bring about the formation of the new leg are not present, or, rather, if present, cannot act. Thus, although the production of new cells from each of the different parts of the leg of a mammal may take place, yet the conditions are unfavorable to the subsequent formation of a new leg out of the proliferated cells. We should not infer that this power does not exist, but that under the conditions it cannot be carried out. The assumption that physiological regeneration is the forerunner of restorative regeneration, in the sense that historically the former preceded the latter and furnished the basis for the development of the latter, cannot be shown, I think to be even probable. This way of looking at the two processes puts them, I believe, in a wrong relation to each other. We find both processes taking place in the simplest forms as in the unicellular protozoa, and present throughout the entire animal kingdom without any connection, excepting so far as they both depend on the general processes of growth characteristic of each organ and of each animal. This leads us to consider the general question of regeneration in its relation to the phenomena of growth.

REGENERATION AND GROWTH

It has been pointed out in several cases in which external factors influence the growth of a plant, or of an animal, that the same factors play a similar part in the regeneration. The action of gravity on the growth of plants has been long known, and that it is a factor in the regeneration of a piece of a plant has also been shown. The only animal in which gravity has been definitely shown to be an important factor during growth is antennularia, and it has been found that gravity is also a factor in the regeneration of the same form. Not only is this influence shown in the growth of the new part that has developed, but the same influence seems to be one of the factors that determines where the new growth takes place. This latter relation is known in only a few cases, for instance in plants, according to VÖchting, and in antennularia, according to Loeb, so that, until further evidence is forthcoming, it is best not to extend this generalization too far; but it seems not impossible that it may be generally true. How an external factor may determine the location of new growth, as well as the subsequent development of the new part, we do not know at present.

In regard to the internal factors that influence the growth and the regeneration of new parts, we are almost completely in the dark. In cases of hypertrophy of the kidney, etc., the evidence seems to show that a specific substance, urea, that is normally taken from the blood by this organ may, if present in more than average amounts, excite the cells to greater activity and to growth, but whether the urea itself does this directly, or only indirectly through the greater functional activity of the cells, has not, as we have seen, been ascertained. That growth is influenced by internal factors can be shown, at least in certain cases, even although we cannot refer to the definite chemical or physical factors in the process. Some experiments that I have made on the tails of fish show very clearly the action of an internal factor. If the tail of fundulus is cut off obliquely, as indicated by the line 2-2 in Fig. 40, A, new material appears in a few days along the outer cut-edge. It appears to be at first equal in amount along the entire edge. As the material increases in width, it grows faster over

that part of the edge that is nearer the base of the tail (Fig. 40, C). This growth continues to go on faster on the lower side, until the rounded form of the tail is produced. If we make the oblique cut so that the part nearer the base of the tail is on the upper side, the result is the same in principle; the upper part of the new material grows faster than any other part. If we make two oblique cuts on the same tail, as shown in Fig. 40, D, or as in E, the new part grows faster in each case on that part of the cut-edge that lies nearer the base of the tail. These results may be supposed to be due to the better nourishment of the new tissues nearer the base of the tail; but it is not difficult to show that the difference in the rate of growth over different parts of the cut-edge is not due to this factor. If, for example, we cut off the tail of one fish squarely near the outer end, as shown in Fig. 40, F, 1-1, and the tail of a second near the base of the tail, as shown in Fig. 40, F, 2-2, and of a third by an oblique cut that corresponds to a cut extending from the upper side of the cut-edge of the tail of the first fish to the lower cut-edge of the tail of the second fish, as shown in Fig. 40, F, we find that the rate of growth over the first and second tails is about the same as that of the lower side of the third tail. In other words, the maximum rate of growth that is possible for the entire oblique edge is carried out only near the lower edge, and the growth of the rest of the new material is held in check. By means of another experiment a similar phenomenon can be shown. If the bifurcated tail of a young scup (Stenopus chrysops) is cut off by a cross-cut (Fig. 40, G, 1-1), it will be found that at first the new material is produced at an equal rate along the entire cut-edge; but it soon begins to grow faster at two points, one above and the other below, so that the characteristic swallow-tail is formed at a very early stage (Fig. 40, H) and before the new material has grown out to the level of the notch of the old tail. If the tail of another individual is cut off by an oblique cut (Fig. 40, G, 2-2), we find, as shown in Fig. 40, I, that at two points the new tail grows faster, but the lower lobe faster than the upper one.

These results show very clearly that in some way the development of the typical form of the tail influences the rate of growth at different points. The more rapid growth takes place in those regions at which the lobes of the tail are developing. In other words, although the physiological conditions would seem to admit of the maximum rate of growth over the entire cut-edge, this only takes place in those parts that give the new tail its characteristic form. The growth in other regions is held in check. The same explanation applies to the more rapid growth at that part of an oblique cut that is nearest the base of the tail, for by this means the tail more nearly assumes its typical form.

These results demonstrate some sort of a formative influence in the new part. We can refer this factor at present only to some structural feature that regulates the rate of growth. We find here one of the fundamental phenomena behind which we cannot hope to go at present, although it may not be beyond our reach to determine in what way this influence is carried out in the different parts. This topic will be more fully considered in a later chapter.

Another illustration may be given from certain experiments in the regeneration of Planaria lugubris. If the posterior end is cut off just in front of the genital pore, as indicated in Fig. 41, new material develops at the anterior cut-edge, and in a few days a new head is formed out of this new material. A new pharynx appears in the new tissue immediately in front of the old part. It lies, therefore, just behind the new head. The proportions of the new worm are at this time very different from those of a typical worm, since the head is much too near to the new pharynx and to the old genital pore. New material is now produced in the region behind the head and in front of the pharynx, so that the head is carried further forward until the new worm has fully assumed the characteristic proportions. As the new head is formed the old part loses its material, so that it becomes flatter and narrower, and if the worm is not fed the old part may lose also something of its former length. If the worm is fed, however, as soon as the pharynx develops the old part loses less and the new part grows forward more rapidly. The most striking phenomenon in the growth of the new worm is the formation of new material in the region behind the head. The result of this growth is to carry the head forward and produce the characteristic form of the animal. This change is all the more interesting since the growth does not take place at a free end, but in the middle of the new material. It is only by the formation of new material in this region that the head is carried to its proportionate distance from the pharynx. It appears that in some way the growth is regulated by influences that determine the form of the new organism.

Another experiment on the same animal gives also a somewhat similar result. If a worm is cut in two obliquely (Fig. 21, B) and the regeneration of the posterior piece is followed, it is found that the new material appears at first evenly along the entire cut-surface. It then begins to grow faster on one side (Fig. 21, b), and a head appears in this region with its axis at right angles to the cut-edge. As the head grows larger the growth is more rapid on one side, and as a result the head is slowly turned forward (Fig. 21, b). This more rapid growth on one side brings the new head finally into its typical position with respect to the rest of the piece. The end result of these changes is to produce a new worm having a typical form. If the oblique cut is made behind the old pharynx, as in Fig. 22, A, the new pharynx that appears in the new material along the cut-edge lies obliquely at first, indicating that the new median line is very early laid down in the new part, and connects the middle line of the old part with the middle of the new head. As the region behind the new head grows larger and broader the pharynx comes to lie more and more in an antero-posterior direction, and finally, when the new part is as broad as the old,[62] the pharynx lies in the middle line of a symmetrical worm.

These results show that the new growth may even take place more rapidly on one side of the structural median line than on the other, and on that side that must become longer in order to produce the symmetrical form of the worm. Here also we find that a formative influence of some sort is at work that regulates the different regions of growth in such a way that a typical structure is produced. The more rapid growth on one side is, however, in this case clearly connected with the relatively smaller development of the organs on that side, and perhaps this same principle may explain all other cases. If so the phenomenon appears much less mysterious than it does when the growth is referred to an unknown regulative factor.

DOUBLE STRUCTURES

A structure that is single in the normal animal may become double after regeneration, and in some cases the special conditions that lead to the doubling have been determined. Trembley showed that if the head of hydra is split lengthwise into two parts, each part may complete itself and a two-headed form is produced. If the posterior end of a hydra is split, an animal with two feet is made. It is true that the two-headed forms may subsequently separate after several weeks into two individuals, and even the form with two feet may lose one of them by constriction, as Marshall and King have shown. Driesch has produced a tubularian hydroid with two heads by splitting the stem partially into two pieces. Each head is perfect in all respects, and although each has fewer tentacles than

the head that regenerates from an undivided stem, yet the number of tentacles on each head is more than half the average number. This is connected apparently with the fact that the circumference of each half is greater than half the circumference of the original stem. Planarians with double tails, produced by partial splitting, have been described by DugÈs and by Faraday, and it has also been shown that by partial splitting of the anterior end of the worm two heads can be produced. Van Duyne, Randolph, and Bardeen and I have obtained the same result. Each half completes itself on the cut-side and produces a symmetrical anterior end. If one of the heads is cut off, it will be again regenerated. If the heads are united very near to the trunk, as in Fig. 42, A, they may never grow to the full size of the original head, as I have found; but if the pieces have been split posteriorly, so that each head has a long anterior end, then each one may become nearly as large as the original head (Fig. 42, B). We see in these cases the influence of the region of union on the growth of the new part. If the new part is near the region of attachment, the smaller size of the latter restrains the growth of the new head; but if the region of union is farther distant, the head may grow more nearly to its full size despite the influence of the region of union. King has found in the starfish that if the arm is split lengthwise, each half may complete itself laterally and a forked arm result. An additional entire arm may be formed by splitting the disk partially in two between two arms. If the cut-edges do not reunite a new arm will grow out from each cut-surface (Fig. 38, E). In this case the development of the new arm cannot be accounted for on the assumption that the typical form completes itself, since a sixth arm cannot be supposed to be a typical structure in the starfish. The result must depend on other factors, such as the presence of an open surface in a region where the cells have the power of making new arms.

Barfurth has been able to produce a double tail in the tadpole by the following method: A hot needle is thrust into one side of the tail, so that the notochord and the nervous system are injured. The tail is then cut off just posterior to the region injured by the needle. A new tail grows out from the cut-end, and also in some cases another tail grows out at the side where the notochord was injured by the needle. The injury to the notochord and the removal of tissue immediately about it leads to a proliferation of cells, around which other tissues are added and the new tail produced.

Lizards with double tails have often been described,[63] and it now appears that all these cases are due to injuries to the normal tail. Tornier has succeeded, experimentally, in producing double and even triple tails. If the end of the tail is broken off, and the tail is then injured near the end, two tails may regenerate, one from the broken end and one from the region of injury (Fig. 43). Under natural conditions this might occur if the tail were partially bitten off and the end of the tail lost at the same time. A regenerated tail may produce another tail if it is wounded. A three-tailed lizard may be made by cutting off the tail and then making two injuries proximal to the broken end. Two of the new tails may be included in the same outer covering if they arise near together, as shown in Fig. 43, B. Lizards with two or three tails may be produced in another way. If the tail is cut off very obliquely, so that two or three vertebrÆ are injured, there arises from each wounded vertebra a cartilaginous tube that forms the axis of a new tail. Tornier thinks that the regeneration is the result of overnourishment of the region where the injury has been made, but this does not seem in itself a sufficient explanation. Tornier has also been able to produce, experimentally, double limbs in Triton cristatus in the following way: The limb is cut off near the body, and, after the cut-end has formed new tissue, a thread is tied over the end in such a way that it is divided into two parts. As the new material begins to bulge outward it is separated into halves by the constricting thread, and each part produces a separate leg (Fig. 43, D). The soles of the two feet in the individual represented in Fig. 43, D, are turned toward each other. The femur is bifid at its outer end, and to each end the lower part of one leg is attached. The bones in this part are fused together at the knee, so that only the foot portions can be separately moved.

The same method used to produce double tails in the lizard can also be used to produce double legs. The femur is broken in the vicinity of the hip-joint, and the soft parts are cut into over the break. Then, or better somewhat later, the leg is amputated below the broken part. A new limb regenerates from the cut-end, and at the same time another limb grows out from the broken femur (Fig. 43, C). The same result is reached if the femur has a slit cut into it in the region of the hip-joint, so that it is much injured. Later the leg is cut off below the place of injury. A double leg is the result.

Feet with supernumerary digits can also be produced by artificial wounds. If the first and second and then the fourth and fifth toes are cut off, as indicated by the lines in Fig. 43, E, so that a part of the tarsus and a part of the tibia and fibula are cut away (the third finger being left attached to the remaining middle portion), more toes grow out from the wounded surface than were removed, as shown in Fig. 43, F. A similar result may be obtained in another way. If the first and second toes are cut off by an oblique cut (Fig. 43, G), and then after the wound has healed the third, fourth, and fifth toes are also cut off by another oblique cut (a part of the tarsus being removed each time), more toes are regenerated than were cut off[64] (Fig. 43, H).

Tornier suggests that the double feet that are sometimes formed in embryos—even in the mammalia—have resulted from a fold of the amnion constricting the middle of the beginning of the young leg, in the same way as is brought about artificially by tying a string over the growing end of the regenerating leg of triton.

In many of these cases, in which the double structure is the result of splitting the part in the middle line, the completion of the new part is exactly the same as though the parts had been entirely separated. The only special problem that we meet with in these instances is that this doubling is possible while the piece remains a part of the rest of the organism. This shows that there is a great deal of independence in the different parts of the body in regard to their regenerative power, and that local conditions may often determine the formation of double structures.

It has been shown during the last decade that double embryos may be produced artificially by incomplete separation of the first two blastomeres. Driesch, Loeb, and others have demonstrated that if the first two cells of the egg of the sea-urchin be incompletely separated, each may produce a single embryo and the two remain sticking together. Wilson has shown in amphioxus that the same result occurs if the first two cells are partially separated by shaking. Schultze has shown in the frog that if at the two-cell stage the egg is held in an inverted position, i.e. with the white hemisphere turned upwards, each blastomere gives rise to a whole embryo—the two embryos being united, sometimes in one way, sometimes in another, as shown in Fig. 63. In this case it appears that the results are due to a rotation of the contents of each blastomere, so that like parts of the two blastomeres become separated. In the egg of the sea-urchin, and of amphioxus, gravity does not have a similar action on the egg, but the results seem to be due to a mechanical separation of the blastomeres. These cases of double structures, produced by the segmenting egg, appear to belong to the same category as those described above for adult forms—especially in those cases where pieces regenerate by morphallaxis.

In connection with the production of double structures there should be mentioned a peculiar method of formation of new heads, first discovered by Van Duyne in a planarian. He found that if the animal is cut in two in the middle line, the halves being left united only at the head-end, as shown in Fig. 44, D, C, there may appear one or two new heads in the angle between the halves. I have repeated this experiment with the same result, and have found that it may also occur when only a piece is partially split from the side of the body, as shown in Fig. 44, B. In Van Duyne’s experiment the two new heads do not appear unless the cut extends far forward, but if the division extends into the region between the two eyes there may be formed, as I have found, a single eye on each side that makes a pair with the old eye of that side (Fig. 44, A). It is evident in this case that each head has completed itself on the cut-side, the completion including the eye and the side of the head also with its “ear-lobe.” The result, in this case, is the same as though the pieces had been completely cut in two. If the cut does not extend quite so far forward there are usually formed one or two heads near the angle, each with a pair of eyes and a pair of ear-lobes (Fig. 44, C). Sometimes a single head develops in the angle itself (Fig. 44, D), and it is difficult to tell whether it belongs to one or to the other side, or whether it is common to both sides. Van Duyne spoke of the double and single head of the latter kind which he obtained as heteromorphic structures in Loeb’s use of the term. According to this definition, heteromorphosis is the replacement of an organ by one that is morphologically and physiologically unlike the original one, but this statement has been made to cover a number of different phenomena. The examples of heteromorphosis that Loeb gives by way of illustration of the phenomenon are: the production of a hydranth on the aboral end of tubularia, and the formation of roots in place of a stem in antennularia, etc. The formation of the heads in the angle in planarians does not appear to me to belong in this category. It seems rather that the phenomenon is of the same sort as the formation of a new head at the side of a longitudinal piece, and if so the new heads in the angle are, therefore, in their proper structural position for new heads belonging to the posterior halves. Even if it should prove true that a single head may develop exactly in the angle itself, and belong to both sides, it can be interpreted by an extension of the same principle.[65] The position of this median head turned backward suggests an obvious comparison with the production of the heteromorphic head in Planaria lugubris, but a closer examination will show, I think, that the two cases are different. The heteromorphic head is produced only when the head is cut off close behind the eyes. If cut off slightly behind this region, a posterior end is generally formed. But in the worms split lengthwise the head in the angle may be formed at a level much farther posteriorly than the eyes. If the split extends into the head, then the eyes that develop are the supplements of those of the old part. Our analysis leads, therefore, to the conclusion that the heads, or parts of heads, in the split worms are not heteromorphic structures but supplementary heads.