Thomas Hunt Morgan

By uniting parts of the same or different animals, or of plants, there is given an opportunity of studying a number of important problems connected with the regeneration of the grafted parts. Trembley’s experiments in grafting pieces of hydra are amongst the earliest recorded cases of uniting portions of different animals, although in plants the process of grafting has been long known.[77] Trembley found that if a hydra is cut in two, the pieces can be reunited by their cut-surfaces, and a complete animal results. No regeneration takes place where the union has been made. He also succeeded in uniting the anterior half of one individual with the posterior half of another individual, and again produced a single individual. He failed to obtain a permanent union between different species.

More recently, Wetzel has carried out a number of different experiments in uniting pieces of hydra. He found that if two hydras are cut in two, the two anterior pieces may be united by the aboral cut-surfaces (Fig. 46, B), and the two posterior pieces may also be united by their oral cut-surfaces (Fig. 46, A). The fusion of these “like-ends” takes place as readily as when unlike ends are brought in contact, as in Trembley’s experiments. Subsequently, however, regenerative changes take place. When, for instance, two anterior pieces are united by their aboral ends, there develop after two or three days one or two outgrowths, at or near the line of union, that become new feet, and the two individuals may subsequently separate. When two posterior pieces are united by their oral surfaces, a double circle of tentacles generally develops, one on each side of the line of union. The pieces then pinch apart and produce two hydras.[78] In another experiment the head and a part of the foot were cut from a hydra, and the head was turned around and grafted by its aboral surface upon the aboral surface of the middle piece. Another animal was cut in two in the middle, and the posterior half was grafted by its oral end to the oral end of the middle piece. In this way a new, artificial individual was made, as shown in Fig. 46, C, with the middle part of the body in a reverse direction as compared with the orientation of the two end-pieces.[79] The union of the three pieces was so perfect that not even a swelling or a constriction indicated the places of fusion. After six days a normal bud appeared at the region of union of the posterior and middle pieces, that gave rise to a new hydra, which separated after a few days. The compound animal was healthy and ate many daphnias. It was kept under observation for twenty-four days, and appeared normal, giving off several more buds.

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Fig. 46.—A. Two posterior pieces of hydra united by their oral ends. B. Two anterior pieces of hydra united by their aboral ends. C. A “long hydra” made by uniting three pieces; the middle piece reversed. D. After Peebles. Two posterior pieces of brown hydra united by oral ends, and one cut off near union. A new anterior end developed from the cut, aboral surface. F. After Peebles. Union of a nutritive and a protective polyps of hydractinia. Subsequently former cut off at line, 1-1. E. Union of two posterior pieces of hydra by oral ends. Subsequently one piece cut off at line, 2-2. E¹. New head regenerated in region of union, and a foot from aboral cut-end. E², E³. Fusion of two parts with a single hydra.

In other experiments of this same sort a foot generally developed where the two aboral surfaces came together, and the head-end separated from the rest of the piece. In another case a mouth and tentacles appeared at the place at which the oral ends had united.

In a different kind of experiment, the anterior ends of two hydras were cut off and united by their aboral surfaces; then one of the components was cut in two, just back of the circle of tentacles. After five days two short, hook-like processes appeared at the cut, oral end. They produced a foot, by means of which the animal fixed itself. In this case it will be seen that a foot developed from an oral end. The result might not in itself be considered sufficient to show whether the development of a foot at the oral end of a piece is due to the influence of the other component, or is simply a case of heteromorphosis having no connection with the presence of the other component. Since heteromorphosis has never been observed in isolated pieces of hydra, the probability is that the result is in some way connected with the presence of the other component. Peebles has made a number of experiments, in which special attention was paid to this point. Fifteen anterior pieces were united in pairs by their aboral cut-surfaces, and then one component was cut in half, leaving an exposed oral end. Out of this number five pieces formed a new head at the cut-surface, and the pieces became attached by a foot, that developed at the region of union. Two others did not regenerate at the cut-surface, but became fixed as before, and neither regenerated nor became fixed at the cut-end. Three became attached at the cut, oral surface, but none of these developed a characteristic foot. The result shows, nevertheless, that some influence was present that inhibited the development of a mouth and tentacles at the oral cut-end, since these always develop in isolated pieces. In another series of experiments posterior ends were united by their oral surfaces, and then one of the two pieces was cut in two (Fig. 46, E). A new hypostome and tentacles developed at the region of union, and a foot at the aboral cut-surface, as shown in Fig. 46, E¹. An organism, with one mouth and a circle of tentacles, and two bodies and two feet, resulted. The bodies soon began to fuse together (Fig. 46, E²) into a single one, and when the fusion had extended to the region of the feet, they also fused into a single structure (Fig. 46, E³), so that a single hydra was produced.

In another experiment, twenty-two posterior ends were united in the same way, and then one of the two components was cut in two. In five cases a single head developed on the aboral end of the smaller piece (Fig. 46, D). It is evident in this case that the union of the two pieces has been a factor in bringing about the development of an aboral head. Another of the grafts produced an aboral head, and also one in the region of union. The remaining sixteen grafts produced new heads, if they developed at all, only in the region of union. Peebles states that the regeneration of aboral heads takes place only when one component is cut off near the region of union of the two pieces.

In general, it may be stated in regard to these experiments in hydra that when pieces are united in the same direction, that is, by unlike surfaces, a single individual is formed and no regeneration takes place where the union has been made, but when like surfaces are brought together, although perfect union may result, a process of regeneration takes place later, at or near the line of union. Even the presence of cut-surfaces at one or both cut-ends of the united components does not generally affect the result, although, in a few cases, it may change it, in so far that heteromorphic regeneration may take place from one piece. This sometimes leads to a suppression of regeneration at the line of union. The experiments do not show, perhaps, conclusively whether the heteromorphosis of the smaller component is due to the polarity of the larger component effecting a change in the smaller one, or whether the closing of the oral end of the smaller component (by its union with the other) brings about the result. All things considered, it seems to me that the larger component has directly influenced the other.

King has found that if two posterior pieces of hydra are united by the oral cut-surfaces, and then after they have fused both pieces are cut off near the line of fusion, there develops from the small piece a single hydra, with a foot at one end and tentacles at the other. If only one of the pieces is cut off near the line of fusion, a new head develops from its oral surface, as Peebles had found. If two anterior ends are united by their aboral cut-surfaces, and then later both are cut off near the line of fusion, a single hydra develops from the small, double piece. If one of the components is cut off near the line of union, a foot develops from the oral cut-end. If in any of the cases the cut is made some distance from the line of union, then each cut-surface develops its typical structure. These experiments leave no doubt as to the influence of the larger piece on the smaller one. Especially interesting is the formation of one individual from two short pieces united in opposite directions. In this case we must suppose that one piece has the stronger influence on the combination (perhaps because it is a little larger), and determines the polarization of the other piece.

King finds that when two posterior pieces are united by their oral ends, regeneration of one or of two heads often takes place at the line of union (Fig. 47, B, B¹, B²), as Wetzel had found. If a dark green individual is united to a light green one, it can be seen that in many cases the new heads are formed by both components, as shown in Fig. 47, B¹. Later one of the posterior ends is absorbed, and the halves may then separate (Fig. 47, B¹, B²). If a number of pieces are united, as indicated in Fig. 47, E, a number of heads may be formed, and one or more of these may have a double origin. No evidences of separation of the pieces was observed in cases of this sort.

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Fig. 47.—After King. A. Hydra split in two, hanging vertically downwards. Later the halves completely separated. B. Two posterior ends united by oral surfaces. B¹. Same; it regenerated two heads, each composed of parts of both pieces. B². Absorption of one piece leading to a later separation of halves. C. Two posterior ends united by oblique surfaces. Later one piece partially cut off, as indicated by line. C¹. Later still, two heads developed, one at N, the other at M. D. Similar experiment in which only one head developed, at M. E. Five pieces united as shown by arrows. Four heads regenerated, one being composed of parts of two pieces.

In one experiment two posterior pieces were united by oblique surfaces, as shown in Fig. 47, C, and one of the two was afterwards cut across, as indicated by the cross-line. The subsequent regeneration that took place is shown in Fig. 47, C¹. A head, composed of parts of both pieces, developed at the cut-surface M, and another in the region N in Fig. 47, C, composed of material of one component. In another case, shown in Fig. 47, D, a head developed only at the cut-edge, but it was made up of material from both components.

A series of grafting experiments of another sort has been made

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Fig. 48.—After Rand. A. Head of Hydra cut off. After eight days. A¹. Same after thirteen days. Three tentacles misplaced. A². Same after eighteen days. A³. Same after twenty-one days. Misplaced tentacles absorbed. B. Anterior end of Hydra fusca, grafted upon side of body of another individual. Half an hour after operation. B¹. Same after four days. B². Same after thirty-eight days. B³. Same, foot-region after forty-nine days. B4. Same after separating. Fifty-second day.

by Rand. A part of one hydra is grafted upon the side of another one in the following way. A groove is scratched in a film of soft paraffine covering the bottom of a dish filled with water. Another groove is made at right angles to the first one, and opening into it. A hydra (the stock) is placed in the first groove, and a wound made in its side with a knife. Another hydra is cut in two, and one piece (the graft) placed in the other groove, and its cut-surface brought into contact with the wound in the side of the first individual. If the operation is successful the exposed surfaces of the two hydras quickly unite, and the combination may be taken out of the groove. If the piece grafted on the stock included about the anterior half of a hydra, a two-headed animal results, as shown in Fig. 48, B. Although the graft has been united to the side of the stock, it soon assumes an apparently terminal position (Fig. 48, B¹). This is due to the graft sharing with the anterior end of the stock the common basal portion of the stock. A slow process of separation of the two anterior ends now begins, brought about by a deepening of the angle between the halves (Fig. 48, B²). This leads ultimately to a complete separation of the two individuals (Fig. 48, B³, B4). Each may get a part of the original foot, or a new foot may arise on the graft as the division approaches the base.

In other experiments only a small part of the foot-end was cut from the animal that served as the graft. The long anterior piece was grafted as before upon the side of the stock. After the two had united, the graft was cut in two, leaving a part of the graft attached to the stock. The part regenerated tentacles, and in two cases subsequently separated from the stock as in the first experiment. In a third case the graft was absorbed by the stock as far as the circle of new tentacles, but its subsequent fate was not determined. In a fourth case the graft did not regenerate its tentacles, and was completely absorbed into the wall of the stock. The smaller the piece that is grafted on the stock the greater the chance that it will be absorbed, and furthermore short, broad rings are more likely to be absorbed than long, tubular pieces of the same volume.[80]

Rand’s results show in general that when hydras are grafted together they regain the typical form in one of two ways,—either by separation into two individuals, or by the absorption of the smaller into the larger component. In the former case the result is brought about in the same way as when the anterior end is partially split in two and the halves subsequently separate. When the graft is absorbed it is not clear whether the absorbed piece disappears or, as seems not improbable, forms a part of the wall of the stock.

It is important to notice the difference between lateral buds and lateral grafts. The buds separate in the course of four or five days by constricting at the base, but this never happens in lateral grafts. Rand has also made some experiments with buds. He cut off the outer oral end of a bud, and grafted it back upon the stock in a new place. It did not separate from the stock as does a bud, but by a slow process of division it was set free in the same way as are lateral grafts. The proximal end of the bud, which was left attached, developed tentacles at its free end, constricted at its base, and was set free. The separation was, however, somewhat delayed.

In another experiment a bud was split in two lengthwise, and the cut was extended so that the body of the parent was separated into two pieces. Twenty-four hours later it was found that each half-bud had closed in, and was much larger than when first cut. The half-bud, that was attached to the posterior end of the anterior piece, was constricting at its base, and subsequently it separated at its point of attachment. The other half of the bud, that had been left attached to the anterior end of the posterior piece, had swung around, so that its long axis corresponded to that of the posterior, parental piece. At first a slight constriction indicated the line of union of the two, but later this disappeared and a single hydra resulted. Whether the difference in the fate of the two half-buds is connected with their different polar relations to the parts of the parent, or is due to some other difference in the absorbing power of the anterior and posterior pieces, is not known.

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Fig. 49.—After Peebles. A. Grafting in Tubularia mesenbryanthemum. A small piece of the stock taken from the region near the base, and grafted in a reversed direction on the oral end of a long piece. B. Same with distal tentacles in small piece, and proximal tentacles in large piece (modified from Peebles). C. Same. Formation of hydranth (original). D. Like A. Both pieces produce hydranths. E. Protrusion of hydranths of last. F. Piece of oral end cut off, turned around and grafted on oral end of long piece. A single hydranth produced. Distal tentacle from both components. G. A short piece from distal (oral) end of long piece cut off, and grafted by its proximal end to proximal end of the same long piece.

Tubularia is not so well suited as hydra to show the influence of grafting on the united parts, since pieces of tubularia produce hydranths, both at the oral and aboral ends, although the latter hydranths take longer to develop. Peebles has shown, nevertheless, that grafting has an influence on the behavior of a piece. In order to show that the polarity of a small piece could be affected by a larger piece, the following experiment was carried out. After cutting off the old hydranth from the end of a stem, a short piece was then cut from the distal end of the same stem, turned around, and its oral end brought in contact with the oral end of the original piece, as indicated in Fig. 49, F. The two pieces, being held together for a few minutes, stuck together and subsequently united perfectly. From eighty-eight pieces united in this way the following results were obtained. Thirty-six formed a single hydranth at the end at which the grafting had been made. The distal row of tentacles appeared in the smaller reversed component, the proximal row in the larger piece (Fig. 49, B). The new hydranth pushed out later through the perisarc of the smaller piece (Fig. 49, C). In this experiment the smaller component was shorter than the average length of the hydranth-forming region. In two cases, in which the smaller component was larger, both circles of tentacles appeared in this piece. In six of the experiments the tips of the proximal tentacles arose from a part of the wall of the smaller piece, hence these tentacles had a double origin (Fig. 49, F). In five of the unions the smaller as well as the larger component produced a hydranth; the two were stuck together by their oral ends (Fig. 49, D, E). The remaining four unions gave somewhat different results. In three of these the smaller piece produced only a part of a hydranth that remained sticking to the end of the hydranth formed by the larger component. In the thirty-six cases in which the minor component took part in the formation of the single hydranth, the influence of the larger component was shown not only in reversing the polarity of the smaller component, although this might in part be accounted for by the closing of the oral end of the smaller piece, but also in the time of development, since the hydranth appeared sooner than does the aboral hydranth and at the same time as does the oral hydranth.

In another series of experiments, a short piece was cut from the basal end of a long piece (three to four centimetres) and brought forward and grafted in a reversed position on the anterior end of the same long piece (Fig. 49, A). Of five unions of this sort, one produced a hydranth in each component, neither being reversed. Another of the pieces produced a hydranth partly out of each component (and at the same time another at the aboral end of the large piece). The other two pieces produced a single hydranth, a part of which came from the minor component and appeared before the aboral hydranth on the aboral end of the larger piece. This last result shows that the small piece from the basal end has been affected by the oral end in such a way that it develops more rapidly than it would have done had it remained a part of the basal end.

In a third series of experiments a short piece (about a half of a millimetre) was cut from the anterior end of a long piece (one and five-tenths to two centimetres) and grafted in a reversed position on the posterior end of the same long piece (Fig. 49, G). In four cases a hydranth developed only at the oral end of the long piece and none from the aboral end or from the short piece. Eight unions produced, however, in the region of the graft, a hydranth formed partly by each component. Later another hydranth developed at the oral end of the larger piece. The latter results are not convincing, but they may show that the small piece has hastened the development of the hydranth at the aboral end.

Peebles has also made some experiments in grafting pieces of different members of the colonies of hydractinia and podocoryne. The colony of the former is made up of three different kinds of individuals: the nutritive, the reproductive, and the protective hydroids. A series of preliminary experiments showed that if these individuals are cut into a number of pieces each piece regenerates the same kind of individual as that of which it had been a part. It was also observed that if pieces of the nutritive individuals were allowed to remain quietly on the bottom of the dish they sent out branching stolons, which stuck to the bottom of the dish, and from these stolons there arose later nutritive hydranths that stood at right angles to the surface. When pieces of the same kind of individuals are grafted together, the results are essentially the same as with tubularia. If pieces of different kinds of individuals are united, the opportunity is given of testing the possible influence of one kind on the other. Peebles united a nutritive and a protective polyp by the cut, aboral ends (Fig. 46, E), and after they had grown together one of the polyps was cut off near the region of union, so that a small piece of a nutritive polyp was left attached to a protective polyp. When the piece of the nutritive polyp regenerated, it made a new nutritive polyp. The influence of the protective polyp was not apparent. If a nutritive and a reproductive polyp are united in the same way, and the latter cut in two near the line of union, a new reproductive polyp develops from the piece left attached to the nutritive polyp. Again there is shown no influence of the one on the other kind of polyp.

Hargitt has also made a number of grafting experiments on other hydroids. His most interesting results are those in which parts of two medusÆ were united by holding their cut-surfaces together by means of bristles passing through the individuals. Hargitt also finds that while in certain hydroids it is possible to bring about a union of oral with oral end, or aboral with aboral, or oral with aboral end of the same species,[81] yet a permanent union between different species cannot be brought about. These results are in agreement with those of a number of writers who have recorded the difficulty or impossibility of uniting parts of different species of hydra. In a few instances it has been possible to unite temporarily a piece of a brown hydra with a piece of a green one,—as I have also seen accomplished,—yet the pieces subsequently separate. Wetzel succeeded in obtaining better results with two species of brown hydras, Hydra fusca and Hydra grisea. In one experiment the head of Hydra grisea was grafted on the body (from which the head had been cut off) of Hydra fusca. After five hours the pieces seemed to have united. Later a constriction appeared at the place of union, and the head-piece produced a foot near the line of union, and the posterior piece produced a circle of tentacles at its anterior end. Eight days later, when the animal was being killed, it fell apart into two pieces. It was observed that during the period of union a stimulus to one piece was not carried over to the other. Wetzel’s results seem to show that pieces of these two species of hydra unite at first, when brought together, as perfectly as do pieces of the same species, but the union never becomes permanent, a constriction appearing later at the line of union, and the pieces separating in this region. These results indicate, it seems to me, that the factors that bring about the first union are different from those that make the grafted pieces one organic whole. Other results indicate that the union of oral to oral end, or aboral to aboral end, while at first as perfect as between unlike surfaces, nevertheless is less permanent than when unlike surfaces are united; at least, subsequent regeneration is more likely to occur in the former than in the latter, and after this occurs the separation of the individuals often takes place. It seems, moreover, not improbable that a more permanent union results when similar regions are united by unlike surfaces, than when the union is at different levels. If, for instance, the anterior half of one hydra is united to the posterior half of another individual, the union is generally permanent; but if one or both of the pieces are longer than half the length, so that a “long animal” results, new tentacles are more often formed at the oral end of one component, and the parts subsequently separate. It may be that, at present, the data are insufficient to establish this general rule, and no doubt other modifying influences must be also taken into account; but it is important that attention should be drawn to this side of the subject.

Grafting experiments in planarians have so far been carried out in only the two cases which I have described. In one of these the anterior ends of two short pieces of Bipalium kewense were united (Fig. 50, A). Neither piece produced a head at the region of union. Later the pieces were cut apart by an oblique cut that passed across the line of union (Fig. 50, C), so that each piece retained at its most anterior end (at one side) a piece of the other individual in a reversed position. A head developed at the anterior (and lateral) end of each piece, in such a way that a part at least of the small reversed piece was contained in the new head (Fig. 50, D). In the other case two pieces of bipalium were united by their posterior cut-surfaces. Each piece produced a new head at its free end, and the pieces greatly elongated, but remained sticking together (Fig. 51).

Fig. 50.—A. Two pieces of Bipalium kewense united by anterior ends. B, C. Later stages of same. Line in C indicates how pieces were cut apart. D. Two worms produced by these pieces. All drawn to scale.

A large number of experiments have been made by Joest in grafting pieces of earthworms. The cut-surfaces were held in contact by means of two or three threads passing through the body wall of each piece and tied across, so that the pieces were drawn together and held firmly in that position. Joest found that pieces of the same or of different individuals could be united in various ways, and the union become permanent. If the anterior end of one worm is united to the posterior end of the same, or of another worm, a perfect union is formed, and no subsequent regeneration takes place (Fig. 52, A). Long worms can be made by uniting two pieces, each more than half the length of a worm, or by uniting three pieces, as shown in Fig. 52, C. Short worms can be formed by cutting a middle piece from a worm, and uniting the anterior and posterior pieces (Fig. 53, D). Joest found that when a short worm is made in this way, so that no reproductive region is present, the new worm does not produce new reproductive organs. It is conceivable that new reproductive organs might have been produced either in the old segments, or by the formation of a new reproductive region between the two united pieces, but neither process takes place. In the long worms two sets of reproductive organs, etc., are present. This sort of union is, however, less permanent, as the worms often pull apart.

Joest also united two posterior ends by their anterior surfaces. In many cases no regeneration took place, and, in the absence of a head, the combination is destined to die, although it may remain alive, without food, for several months. When two very long pieces were united by their anterior ends,—only eight segments being removed from each worm,—although perfect union took place at first, later one or two new heads generally developed at the region of union (Fig. 52, D). When only one head developed it did not seem to belong to one of the components rather than to the other, and originated in the new tissue that appeared between the two pieces. These experiments, in which the anterior surfaces of two pieces are united, show also that the new head arises between the two pieces most often, if not exclusively, when the union is in the anterior ends of the worms. This corresponds with what is now known in regard to the development of new heads by isolated pieces, since there is less tendency to produce a head the farther posteriorly the cut has been made. At more posterior levels a tail and not a head is often regenerated, as has been stated, on the anterior cut-surface. This formation of a heteromorphic tail seems to have been suppressed in the pieces united in this region, except in one case,[82] in which it appears, from Joest’s account, that a tail probably regenerated, although Joest speaks of it as a head.

It is more difficult to unite two anterior ends by their posterior cut-surfaces, not because the surfaces refuse to unite, but because the two pieces crawl away from each other and pull apart. In one case, however, union of this sort was brought about.

In all the combinations that have been so far described, the dorsal and ventral surfaces of both components were kept in the same direction, so that the ventral nerve-cord of one piece came in contact and fused with the nerve-cord in the other piece. Sometimes it may happen that the components are not quite in the same position, and the end of one nerve-cord may fail to abut against the other one. In such cases Joest thinks that regeneration is more apt to take place in the region of union, and he has carried out a series of experiments in which the pieces were intentionally united, so that they are not in corresponding positions. It is found that if one piece is turned so that the nervous system lies 90 degrees, or even 180 degrees (Fig. 52, B), from that of the other piece, the union takes place just as when the pieces have the same orientation, except that the ends of the nerve-cords do not unite. Subsequent regeneration from one or from both components generally takes place in the region of union.

It is more difficult to unite pieces of different species of worms, yet Joest has succeeded also in making combinations of this sort. One union between the anterior end of Lumbricus rubellus and the posterior end of Allolobophora terrestris was permanent, and the new worm reacted as a single individual, and lived for eight months. Each piece retained its specific characters, and showed no influence of the other component. By means of a similar experiment we have a way of finding out if one component can influence regeneration taking place from the other piece. Although Joest made only a few observations of this sort, the results show that no such influence is manifested.

By means of grafting it is possible to keep alive small pieces of a worm that would otherwise perish. For instance, pieces of a worm containing only three segments are not capable of independent existence, except for a short time, and even pieces of from four to eight segments die in most cases. It is not possible to unite small pieces of this size directly upon larger pieces, since they will die, ordinarily, as a result of the operation, but larger pieces can be united and then after union has been effected, one of them may be cut off near the place of union. The same result is sometimes brought about accidentally by the worms themselves pulling apart and leaving a small piece of one component attached to the other. Joest found that in several cases these small, attached pieces regenerated. In one case, after two long pieces had pulled apart, a small piece, left by one of the two, regenerated a single new segment with a mouth at its end. In another case, after one of the components had been cut off, leaving two segments attached, a new part of seven segments regenerated.[83] Especially interesting is the case in which two individuals (A. terrestris) had been united to form a long worm. The anterior component extended to within two centimetres of the anus; the posterior piece had had the first four segments removed. Three days later the anterior piece was cut off three segments in front of the region of union. About a month later a small part of eight segments had regenerated from the cut-end (Fig. 53, A). Fifteen days later another new part of thirty-seven segments developed at the end of the first new part (Fig. 53, B). Joest speaks of the first eight segments as a head, and the second simply as a regenerative product. There can be little doubt, I think, that both parts represent a heteromorphic tail. The region from which the regeneration took place would make this interpretation highly probable, and Joest’s figures also indicate that the structure is a tail. The result is very interesting, if my interpretation is correct, as it shows that the major component did not influence the kind of regeneration, although the surface of regeneration was separated by only three tail-segments from the anterior end of the major component.

In another experiment a long animal was made by uniting Lumbricus rubellus (whose posterior third had been cut off) and Allolobophora terrestris (whose first six segments had been cut off). Four days later the two components had torn apart, but a small piece of the anterior worm remained attached to the anterior end of the posterior component. The small piece consisted of the dorsal part of two and a half segments without any ventral part, so that the anterior end of the posterior component was partially exposed. The small piece of lumbricus was much lighter in color, and this difference made it easy to distinguish between the two. In less than a month the small transplanted piece had replaced its missing ventral part, so that the entire anterior surface of the larger component was covered over. The small piece, in addition to regenerating its ventral part of four segments, had also begun to make new segments. After a month and a half six new segments were present (Fig. 52, E), with a mouth at the anterior end.[84] Even after ten months the color of the small piece was strikingly different from that of the major component. The new head had the typical red-brown color of L. rubellus, that forms a strong contrast to the grayish blue color of A. terrestris.[85] The result shows that the color of the regenerated part has not been influenced by that of the posterior component, and this is all the more interesting, as Joest points out, because the small piece that was left after the worms pulled apart was too small to have lived independently for any length of time, and must have derived all its nourishment from the larger piece.

In other experiments pieces of one species were cut from the side of the body and grafted upon the cut-surface of the anterior end (or elsewhere) of another species. In one of these experiments a piece from the side of A. terrestris, that extended over five or six segments, was sewed upon the anterior cut-surface of L. rubellus (from which the anterior five segments had been removed). In about a month new tissue appeared on the ventral side between the two pieces, and a little later a complete head developed, whose dorsal side was made up of the small piece (Fig. 53, C). The grafted piece was dark, and the new, regenerated part light in color and continuous with the brown color of L. rubellus, from which the new part had arisen. It is important to notice that the four segments of the graft are completed by four segments of the new part. After three months the new part had assumed the red-brown color of L. rubellus. The color of the grafted piece had not changed. We see in this case that even the presence of a part of another worm in a regenerating region does not have any influence, at least so far as color is concerned, on the new part, even though its segments supplement some of those of the new part. The new tissue seems to have come entirely from the major component, and to have carried over the color characteristics of the old part.

It has been shown that when two posterior pieces are united by their anterior ends the combination must sooner or later die, since it has no way of procuring food. The question arises: What will happen if one of the two components is cut in two near the place of union? Will a head then develop on the exposed aboral surface, because a head is needed to adapt the worm to its surroundings, or possibly, if it occurred, because the major component exerts some sort of influence on the short, attached piece, as happens in hydra and in tubularia? Both Joest and I carried out an experiment of this sort, and found that a tail and not a head regenerated, as shown in Fig. 16, F. The experiment is, however, insufficient to answer the question, since the region in which the second cut was made is a region from which only a tail (and not a head) arises, even when the oral end of a piece is exposed. In order to avoid this difficulty I carried out another experiment. Two worms had the first five or six segments cut off and the exposed anterior ends of the worms united, as shown in Fig. 16, D. Then one of the components was cut off, leaving three or four segments attached to the anterior end of the other component. Although regeneration began in one case, it did not go far enough to show what sort of a structure had developed, but Hazen, who took up the same experiment, succeeded in one case in obtaining a definite result. At the exposed aboral end of the small piece a head and not a tail developed (Fig. 16, E). At first sight it may appear that the result shows the influence of the major component on the small piece, causing it to produce a head and not a tail at its aboral end, but I think that this conclusion would be erroneous, because it seems much more probable that we have here a case of heteromorphosis, similar to that in Planaria lugubris, and that the result depends entirely on the action of the smaller component. It is hardly possible to demonstrate that this is the correct interpretation, since if a small piece of this size is isolated it dies before it regenerates. The result is paralleled, however, by the regeneration of a tail at the anterior surface of a posterior piece.

The process of grafting has long been practised with plants, but the experiments were made more for practical purposes than to study the theoretical problems involved. VÖchting has, however, carried out a large number of well-planned experiments. He finds that a stem can be grafted upon a root, and a root upon a stem, a leaf upon a stem or upon a root. Even an entire plant can be grafted upon another. The results show, however, in general, that, whatever the new position may be, the graft retains its morphological characters—a shoot remains a shoot, a root is always a root, and a leaf a leaf. VÖchting concludes that there is in the plant no principle or organization that conditions an unchangeable arrangement of the main organs. “The inherited order of the parts, acquired apparently on physiological grounds, may be altered by the experimentator; it is possible for him to change the position of the building blocks within a wide range without endangering the life of the whole.” “It is essential, however, for the success of the experiment that the grafted parts, or tissues, retain their normal orientation. If this condition is not fulfilled there may take place, it is true, a union of the parts, but sooner or later disturbances set in.” VÖchting transplanted pieces in abnormal positions, sometimes reversing the long axis of the grafted piece, sometimes the radial axes, and sometimes both together. In some cases this led to the formation of swellings that interfered with the nourishment but carried with it no further consequences. In other cases the changes went so far that the vital processes were interfered with. At times an incomplete union took place between the parts; at others, even though the first union was perfect, death later ensued.

On the other hand, when similar pieces were grafted with their original orientation, a perfect union took place and the piece became a part of the stock. The results establish, VÖchting claims, that every part and every portion of a part has a polar orientation in one direction, and furthermore, in a body having a radially symmetrical form, there is also a radial polarization; that is, the inner side of each part is different from the outer side of the same surface, even though no such difference is apparent to us. The properties of the tissue-complex rest, in the last analysis, on that of the cells; the properties of the whole being only the sum total of the properties of its elements, so that we may say that every living cell of the root is polarized, not only longitudinally, but also radially; each has a different apical and root pole, a different anterior and posterior pole, and also right and left polar relations. These results, deduced from the experiments in grafting, lead VÖchting to formulate the following rule: “Like poles repel, unlike poles attract.” This rule is the same as the law of the magnet. In fact, VÖchting states that the root and the stem relations show a remarkable resemblance, despite many differences, to a magnet. If the magnet is broken into pieces it may be reunited by bringing unlike poles together, but not by uniting like poles; the same statement holds for the root and the stem.

Exception may be taken, I believe, to parts of VÖchting’s conclusions, especially in the light of the recent experiments in grafting in animals. It is by no means to be granted without further demonstration that the properties of the whole organism are only the sum-total of the action of the individual cells. If, as seems to be the case, the cells are organically united into a whole, the properties of this whole may be very different from the sum of the properties of the individual cells, just as the properties of sugar are entirely different from the sum of the properties of carbon, hydrogen, and oxygen.

The statement that like poles repel and unlike poles attract is, I believe, a conclusion that goes beyond the evidence. The experiments show that like poles do often unite in plants, and this has been abundantly shown to be the case in the lower animals, and even in forms as high as the earthworm and the tadpole. Even if when like poles are united subsequent changes take place, that in some cases, although apparently not in animals, lead to the death of the graft, it by no means follows that this has anything to do with the attraction or repulsion of the parts, but rather with some difficulty in obtaining food, or with the transportation of substances through the plant. In the lower animals we have seen that when like poles are united there is sometimes a stronger tendency to produce new organs at or near the place of union than when unlike poles are united, but it would be going too far, I think, to state that this is due to repulsion of the parts, especially in the sense in which the like poles of a magnet repel each other. It seems to be due rather to the two parts failing to unite into a whole organization, each retaining the same structural basis that it had before grafting, but this is a very different principle from that of an attraction and repulsion of the parts, and the question of the union of the parts appears also to be a different question from that of the organization of the parts themselves.

In the mammals, and in general in all forms in which there is a dependence of the parts on each other, it is impossible to carry out grafting-experiments on the same scale as those described in the preceding pages. The principal difficulties are to make the parts unite, and to supply nourishment and oxygen to the graft. Owing to the dependence of the parts of the body on each other for a constant supply of oxygen and food derived from the blood, as well as for the removal of the waste products, the parts cannot remain alive, or even in good condition, while new connections are being established. For this reason, as well as for others, it would not be possible, for instance, to graft the arm of a man upon another man. The tissue may have the power of uniting even in this case, as is seen when the bone is broken and subsequently reunited, but the difficulty would be in supplying the grafted arm with nourishment, etc., during the long time required for the union to take place. Smaller parts of the body may be successfully grafted, and there are several recorded cases in which parts of a finger, or of the nose, are said to have been cut off and to have reunited after being quickly put back in place. Pieces of human skin may be grafted without great difficulty upon an exposed surface, and it has been said that small pieces succeed better than larger ones, owing, most probably, to their being able to absorb sufficient oxygen, etc., and keep alive until new blood vessels have grown into the grafted piece.

There are a number of old and curious observations in regard to cases of grafting in higher animals. It was found by Hunter and by Duhamel that the spur of a young cock could be grafted upon the comb, when it continued to grow to its normal size. The comb, being richly supplied with blood, furnished the nourishment for the growth of the spur. Fischer transplanted the leg of an embryo bird to the comb of a cock, or of a hen, where it grew at first, but after some months degenerated. Zahn transplanted the foetal femur to the kidney, where it grew for a time, but later degenerated. Bert transplanted the tail of a white rat to the body of Mus decumanus, where it continued alive; but he found that the tail of the field mouse, Mus sylvaticus, did not grow so well on the rat, and the tail of a rat would not unite at all with the body of a dog or of a cat. Bert bent over the tip of the tail of a rat, and grafted the distal end into the skin of the back of the same animal. After the tip had established union with the surrounding tissues, the tail was cut off at its base. The grafted tail remained alive, but did not regenerate at its free end.

There are several cases described by pathologists in which the skin of one mammal has been transplanted to another. The transplantation of the skin of the negro upon a white man has been brought about, but the evidence as to what subsequently happened is contradictory. It appears that while in many instances the transplanted skin has remained alive for a time, yet later it was thrown off by new skin growing under it and replacing it.

Leo Loeb has described a curious instance of grafting pieces of skin of different colors in the guinea pig. If a piece of black skin from the ear of a guinea pig is grafted upon the white ear of another animal, it unites and continues to live, but if a piece of white skin is grafted upon a black ear, it is slowly thrown off and replaced by new black skin that has regenerated around the edge of the graft from the tissue of the black ear.

In the literature of pathology there are many cases described in which parts of the body of mammals, particularly internal organs, have been grafted in unusual regions. The results have not always been the same, for while in some cases it appears that the operation has succeeded, in others the grafted part is subsequently absorbed, and in still other cases the graft may be at first partly absorbed and later begin to grow again. It appears that the establishment of an adequate blood supply is the most important element of success. Ribbert, who has made an extensive and successful series of experiments, has stated that the grafting takes place better when small pieces of an organ are used, since these can draw immediately on the surrounding regions for their oxygen, etc., while larger pieces are found to break down in the interior, owing to the fact that this part is too far removed from the supply of oxygen, food, etc. After the grafted piece has established a blood supply of its own, it may continue to grow. Ribbert transplanted small pieces of different tissues of the rabbit and guinea pig in, and upon the surface of, the lymph glands of the same or of another individual. The lymph gland was chosen because small pieces of tissue can be afterwards easily detected. A small piece of tissue about as large as a pin’s head is cut off from whatever tissue is to be grafted, and as quickly as possible placed in a small cleft made in the lymph gland. After several days, weeks, or months, the gland is removed and the graft examined by means of serial sections.

Most of the experiments were made with “epithelial organs,” and according to Ribbert, if pieces of such organs are composed of epithelium only, they cannot be successfully grafted. For instance, the cells of the cornea can be readily separated from their underlying connective tissue, and can be kept alive in the lymph gland, but the cells diminish in number, show retrogressive metamorphosis in the direction of atrophy, and are finally absorbed. It seems that epithelium by itself cannot extract nourishment from its surroundings. Nothing is easier, however, than to transplant epithelium, if its connective tissue is present. The connective tissue furnishes so good a basis for nourishment that the epithelium not only lives, but may continue to proliferate. Ribbert finds that pieces of skin roll in after their removal. Then a process of growth takes place corresponding to that which follows a wound in the skin. The surface is closed and a small cyst is formed with a central cavity. The epithelium undergoes no changes during the first days or weeks. It remains stratified and shows an active process of cornification and desquamation. Similar results were obtained when pieces of the conjunctiva were transplanted, either under the skin in the anterior chamber of the eye, or in the lymph gland.

A small piece of the lining epithelium of the trachea with its underlying cartilage was also placed in the lymph gland. The epithelium grew, and covered over the wounded surface, forming over it only a single layer of cells. The old many-layered epithelium also became arranged in a single layer.

The wax glands, found in the inguinal folds of the rabbit, were also transplanted. The gland is composed of closed, compressed alveoli, surrounded by large, polygonal, clear cells. Small pieces of a gland, transplanted upon the lymph gland, underwent characteristic changes. The cells of the alveoli were changed into a stratified epithelium; and broken-down cells, and wax, were found in the interior of the alveoli. The central alveoli underwent the greatest change, while some of the peripheral alveoli that were in contact with the lymph gland remained unchanged. It seems that the difference is due to the better nourishment of the outer alveoli. After several months the alveoli swell up and degenerate. Transplanted pieces of the salivary glands also change, the alveoli producing a lining epithelium like that of the transplanted wax gland. The same change was observed in a piece of a salivary gland transplanted in the body cavity.

Small pieces of the liver were cut off and placed in the lymph gland. They did not always grow as well as did the preceding tissues, but often went to pieces. If they healed, the liver tissue often remained unchanged for several weeks. After two or three weeks connective tissue appeared between the peripheral liver cells, separating the cells from each other. The cells grew smaller, their protoplasm disappeared, and they at last disintegrated. Pieces of the gall duct behaved differently. They sometimes showed active growth, leading to the development of numerous branched canals.[86]

Pieces of the kidney, when transplanted, suffered a great change, and were subsequently absorbed. Transplanted pieces of a testis also changed. After six days, Sertoli’s cells and the spermatozoa disappeared. A kind of indifferent cell remained, characterized by clear protoplasm and by a large nucleus. After seventeen days further changes were observed, and later the pieces were completely absorbed. Pieces of the ovary rapidly disappeared, leaving only a mass of interstitial connective tissue.

The connective tissue underwent, in all the transplanted pieces, characteristic changes. The tissue became less dense, the protoplasm and nucleus of each cell enlarged. The cells multiplied, but only very slowly. These changes took place after one or two days. After a month or two the cells became more compact, their processes more numerous, and the nucleus small and long. Later degeneration set in.

Small pieces of bone from the caudal vertebrÆ were also transplanted, care being taken that each piece should contain some of the periosteum and marrow. The bone tissue goes to pieces, but the periosteum and marrow develop further. New bone is formed from the cells of the marrow as well as from those of the periosteum. Finally the entire piece, both its old and its new parts, is absorbed. Pieces of muscles were also absorbed.

These experiments of Ribbert show that transplanted pieces of tissue do not increase in size by growth, but undergo changes which he describes as a return to an earlier condition of development. The abnormal condition of their existence seems to be the cause of this change. The transformation may be due to a change of nourishment, or to a loss of nerve influence, or to lessened functional activity.

These results have a direct bearing on the problem of regeneration. They show that all kinds of tissue may continue to live, and the cells multiply in different parts of the body, but there seems to be nothing in these cases comparable to a regeneration of the entire organ. In the new situation the cells often assume an entirely new arrangement. After a period of activity, a process of degeneration commences, and the piece atrophies. Ribbert thinks that the atrophy is due to lack of nourishment, yet it is not clear how this could be the case, since for the first few weeks after transplantation there is an active growth, and in some cases, as in that of the bone, there is a formation of new, characteristic tissue. It may be that the transplanted tissues can no longer manufacture the substances necessary for their specific growth, and after the materials that have been brought along with them have been used up, the growth of the piece is stopped and its subsequent degeneration begins. It would be interesting to see if pieces transplanted to the same kind of organ as that to which they belong will become permanently incorporated in their new position.

The grafting-experiments that have been described in the preceding pages were carried out with pieces of adult organisms. Somewhat different conditions are present when parts of the developing egg or embryo are united, inasmuch as a process has been started in them that may go on independently, to a certain extent, of the union of the pieces. Born has carried out a large number of experiments in grafting parts of tadpoles of the same species, and also of different species. The union is brought about at the time when the tadpoles are about to leave the jelly membranes. The cut-surfaces are brought in contact and the pieces pushed together and held in place for an hour or two by means of small silver blocks or pieces of wire. The pieces readily stick together, and the union is a permanent one. Before describing Born’s results, it may be well to consider the power of regeneration of young tadpoles. If the tail is cut off a new one is regenerated by the tadpole, but all parts of the body do not have this same power. Schaper found that if a part of the brain, or even the entire brain, is removed, no regeneration takes place. I have found that if the region where the heart is about to develop is cut out from a young embryo, a new heart is not formed.[87] If a tadpole is cut in two across the middle of the body, neither piece regenerates the missing half. Byrnes has found, however, that if the region from which the posterior limb develops is cut out a new limb regenerates. In older tadpoles, Spallanzani found that if the hind limb is cut off it will regenerate, and Barfurth has more recently confirmed this result. The end of the tail that has been cut off from a young tadpole, before the tail has begun to differentiate, may continue alive for several days. It grows larger and flatter, and the V-shaped mesoblastic somites are formed. A slight regeneration even starts at its anterior end, as first observed by Vulpian and later by Born. The notochord and nerve-cord may send new tissue into the new part, and even some of the muscle cells may extend into this part, but the piece dies before regeneration goes any further. If, however, the tail is grafted in a reverse direction on the body of another tadpole, the regeneration may go further and produce a tail-like structure, as Harrison discovered and as I have also seen.

Born found that if the anterior half of one tadpole was united to the posterior half of the same or of another tadpole a single individual was formed which he kept alive in several cases until the time of metamorphosis. If the head of a tadpole is cut off and grafted upon the side of the body of another tadpole, the head will remain alive and continue to develop in its new position, and, if well nourished by means of the connecting blood vessels that develop, it may grow to be as large as the head of the tadpole to which it is attached. Similarly, if the tail of one tadpole is grafted upon the side of the body of another tadpole, it also continues to develop, and at the time of metamorphosis, when the normal tail is absorbed, the additional or misplaced tail also shows signs of breaking down. Even the posterior half of one tadpole, if grafted to the ventral side of another, may continue to develop, producing legs, etc.

Born succeeded in uniting tadpoles of different species in several different ways. They were united by their heads or by their ventral surfaces, or longer and shorter tadpoles made by using pieces longer or shorter than a half. In all of these cases there is no regeneration at the place of union, and the internal organ, the digestive tract, nervous system, and blood vessels unite when brought into contact. When pieces are united end to end, like organs unite to like, the nerve-cord with the nerve-cord, digestive tract with digestive tract, segmental duct with segmental duct, coelom with coelom, and although less often, the notochords sometimes join together. The lack of union of the ends of the notochord is explained by its frequent partial displacement at the cut-end, for when the cut is made the notochord, being tougher than the other structures, is often dragged out of place in one or in both pieces, so that the ends do not meet when the pieces are put together. When like organs are brought together the substance of one unites directly with the substance of the other, and if the organ is a hollow one, as is the digestive tract or the nerve-cord, their cavities also become continuous. There is also, Born states, some evidence to show that if similar organs are not brought exactly in contact their ends find each other and unite, and if they do not at first meet squarely they may do so later. When the ends of unlike organs are brought in contact, as, for instance, the nerve-cord and notochord, they do not unite, but connective tissue develops between them. The union of like parts, Born suggests, may be due to some sort of cytotropism, the outcome of a mutual attraction between similar cells like that which Roux has observed between the isolated cells of the segmented egg of the frog. Born thinks that the first rapid union of the pieces is due to the attraction of the ectoderm of one component for that of the other.

Born succeeded also in uniting pieces of the tadpoles of different

species, even when they belonged to different genera. It is found, however, that some of these combinations can be more easily made than others, but it is not clear whether the difference depends upon differences in the sizes of the pieces, or the rate of growth of the ectoderm over the cut-surfaces, or to a deeper-lying lack of affinity between the tissues. A combination of Rana esculenta (anterior) with Bombinator igneus (posterior) was made. The combination lived for ten days, and then showing pathological changes, it was killed. Another combination is shown in Fig. 54, E, in which the anterior part of Rana esculenta was united to the posterior part of Rana arvalis.[88] The blood of the posterior component was driven through the vessels by the action of the heart of the anterior component. The animal lived for seventeen days.

In all these combinations between different species, each developing part retains its specific characters, and, although in several cases one part received its nourishment from the other through the common circulation, yet no influence of one component on the other could be observed.

Harrison has succeeded in keeping an individual made up of two species, Rana virescens and Rana palustris, for a much longer time,—until, in fact, the transformation of a tadpole into a frog had taken place. Each half retained the characteristic features of the species to which it belongs.

The absence of regeneration after the union of the pieces may be attributed, in several cases, to the absence of this power in the region through which the cut has been made; but in other experiments this cannot be the explanation, since the power to regenerate can be shown to exist in the part. This is the case in an experiment carried out by Harrison and repeated later by myself. If the tips of the tail of two tadpoles are cut off and interchanged (Fig. 55, A, B), a perfect union takes place between the two parts, and a single tail develops. Each of the cut-surfaces has the power to regenerate, but the union of the parts has suppressed the regeneration. If, however, like parts are not brought in contact, regeneration may take place in the region of union (Fig. 55, D).

Both Harrison and I have made a number of experiments, in which the end of the tail of a tadpole of one species was interchanged with a similar part of another species. It is found that as the new tail grows larger the ectoderm of the grafted piece is carried out to the tip of the new tail, as shown in Fig. 55, C, and does not cover all the inner tissues that belong to the same piece, the rest of the tail being covered by the ectoderm of the major component. If the tip of the tail is now cut off, as indicated by the line b-b in Fig. 55, C, there are left at the exposed edge two kinds of ectoderm, and from the cut-edge a new tail regenerates, covered in part by each of the two kinds of ectoderm. I made this experiment in order to see if the new ectoderm would show any influence of its dual origin, especially along the line where the two kinds are in contact, but no influence could be detected. In another series of experiments the grafted tail was cut off, as shown in Fig. 55, A, or in Fig. 55, B, or in Fig. 55, C, a-a. In these cases there is left exposed, at the cut-edge, the internal tissues of the two species. The new tail that regenerates is composed in part of material derived from one species and in part from that of the other, but each tissue remains true to its kind, and there is found no evidence of an influence of one on the other (Fig. 55, E). These experiments show that even when the two kinds of tissue regenerate side by side, and unite to form a single morphological organ, there is no influence of a specific kind of one tissue on the other.

Another series of experiments in grafting, similar to one of those made by Joest and myself on the earthworm, has been made by Harrison on the tadpole. I have also later made similar experiments. Two tadpoles are united by their posterior ends, as shown in Fig. 54, A, and a day or two after union one of the tails is cut off near the line of union. There is thus left attached to the end of the tail of one tadpole a part of the tail of the other united in a reverse direction, so that the exposed cut-end is the anterior end of the small piece. There grows out from this cut-end a structure that resembles a tail (Fig. 54, B, C, D). It contains a continuation of the notochord and nerve-cord, that taper in a characteristic way to the end of the new structure. The tail is flat and has a central band of muscle tissue, and a dorsal and ventral fin. The muscles of the normal tail have a characteristic V-shaped arrangement with the apex of the V’s turned forward, but unfortunately in the new tail the muscles are so irregular that it is impossible to make out their arrangement (Fig. 54, D). If the new part is in reality a tail, the V’s ought to stand in the same way as do those in the major component, and opposed to the V’s on the part from which the new material arises. If the new structure is not a tail at all, but a new growth, or even a suppressed trunk, then the V’s should stand as in the small part itself. It has not been possible as yet to obtain a decisive case. Harrison obtained one case in which the arrangement of the muscles in the new part seemed to be more as it should appear if the new part is a heteromorphic tail (Fig. 54, D). Even if this could be shown to be the case, it may be that under the conditions of the experiment the arrangement of the muscles is determined by the use of the tail, although this does not seem very probable. Harrison, after a careful analysis of the question, left it undecided, but seemed more inclined to the view that the result is due to the development of something new rather than a heteromorphic growth. On the contrary I am strongly inclined to believe that the latter is the true explanation. In another way I have been able to bring about the development of the same structure. A small triangular piece is cut from the upper part of the tail, as indicated in Fig. 56, A, one point of the triangle passing through the notochord, or even through the aorta. If the cut-surfaces are kept apart for a few hours, until the exposed end has been covered over by ectoderm, they may not unite afterward, and two exposed surfaces are left,—one at the distal end of the base of the tail, and the other at the proximal end of the outer part of the tail. The latter surface corresponds to that in the grafting-experiment. Regeneration may take place from the two surfaces; both new parts seem to be exactly alike, and both resemble a regenerated tail. The one from the proximal surface of the outer part of the tail contains a notochord, nerve-cord, connective tissue, pigment cells, and muscle tissue (Fig. 56, B). The arrangement of the muscle fibres is generally very irregular, and the characteristic V-shaped arrangement cannot be detected.

In only a few cases have attempts been made to unite two eggs or two very early embryos, although there are a few casual observations[89] in which such a fusion has been observed. The problems that arise in connection with the union of two eggs are full of interest. Each egg has the power of producing an embryo of normal size. If two eggs are united into one, will a single giant organism result, or two organisms? If the former, we must suppose that a new organization is formed of double size. Whether an upper limit of organization exists can only be determined by such an experiment. If two fused organisms result from the fusion of two eggs, it would show the structure of the egg is of such a kind that two organizations cannot readjust themselves into a single one of double size. Moreover, it is important to discover whether any difference exists as to the stage of development at which the union is brought about, for it is conceivable that while a rearrangement is possible at one stage, it might not be at another.

Fig. 56.—A. Tadpole to show where the V-shaped piece is cut from the tail. B. Later stage of same with a new tail-like outgrowth from the anterior end of tail.