Regeneration

LATERAL REGENERATION

Not only does regeneration take place in an antero-posterior direction, but in many animals also at the side. The regeneration of the limb of the salamander is, of course, a case of lateral regeneration in relation to the animal as a whole, but in a longitudinal direction in regard to the limb itself. Lateral regeneration of the limb would take place if the limb was split lengthwise into two parts and one of the parts removed. If the entire salamander were cut in two lengthwise, each half would most certainly die without regeneration, if for no other reason than that the integrity of the median organs is necessary for the life of the different parts. If, however, a planarian is cut lengthwise into a right and left half, each piece will complete itself laterally and make a new worm (Fig. 13½, A-D). Even a narrow piece cut from the side will produce a new worm by regenerating laterally, as shown in Fig. 19, a, b, c. In hydra, also, a half-longitudinal piece produces a new animal, but in this case not by the addition of new material at the side, but by the cut-edges meeting to make a tube of smaller diameter. Subsequently the piece changes its form into that characteristic of hydra.

REGENERATION OF TERMINAL PORTIONS OF THE BODY

In most of the preceding examples the behavior of the larger piece of the two that result from the operation has been described; but there are some important facts in connection with the regeneration of the smaller end-pieces. The leg, or the tail, that has been cut from the salamander soon dies without regenerating. The life of the leg can be maintained only when the part is supplied with certain substances from the body of the animal. It does not follow, of course, that, could the leg or the tail be kept alive, they would regenerate a salamander. In fact, there is evidence to show, in the tail at least, that, although it may regenerate a structure at its anterior end, the structure is not a salamander, but something else. This has been definitely shown in certain experiments with the tail of the tadpole. It is possible to graft the tail of one tadpole in a reversed position, i.e. with its anterior end free, on the tail of another tadpole (Fig. 54, A-D), or even on other parts of the body. Regeneration takes place from the free end, i.e. from the proximal end of the grafted tail. The new structure resembles a tail, and not a tadpole. If it be objected that the experiment is not conclusive because of the presence of the old tail, or of the use of the newly developing part, the objection can be met by another experiment. If, as shown in Fig. 56, A, a triangular piece is cut out of the base of the tail of a young tadpole, the cut being made so deep that the nerve-cord and notochord are cut in two, there develops from the proximal end of the tail a new tail-like structure that is turned forward, or sometimes laterally. In this case the objections to the former experiment do not apply, and the same sort of a structure, namely, a tail, is produced.

In the earthworm also we find some interesting facts connected with the regeneration of the terminal pieces. If one, two, three, four, or five segments are cut from the anterior end, they will die without regenerating. Pieces that contain more segments, six to ten, for example, may remain alive for a month or longer, but do not regenerate (Fig. 3, A, B). That this lack of power to regenerate at the posterior end is not due to the smallness of the piece can be shown by removing from a piece of five segments one or two of its anterior segments. These will be promptly regenerated. Another experiment has shown, however, that if these small pieces can be kept alive for a long time, and also supplied with nourishment, regeneration will take place at the posterior end. If, for instance, a small piece of eight or ten segments has its anterior three or four segments cut off, and is grafted by its anterior end to the anterior end of another worm, as shown in Fig. 3, F, the piece will begin, after several months, to regenerate at its exposed posterior end, but in the one instance in which this experiment has been successfully carried out, a new head, and not a tail, appeared on the exposed free end. The result is not due to the grafting, or to the anterior position of the posterior end, but to some peculiarity in the piece itself. We find the converse of this result in an experiment with the tail region of the earthworm, where the outcome is more clearly seen to be connected with the nature of the piece itself. If a piece less than half the length of the worm is cut off from the posterior end, there is generally formed from its anterior cut-surface, not a head, but another tail (Fig. 2, I). The result is similar to that described by Bonnet for one of the fresh-water annelids. A parallel case to that of the head of the earthworm is found in one of the planarians. If the head of Planaria lugubris is cut off just behind the eyes (Fig. 4, F), there is produced, at the posterior cut-edge of the head, a new head turned in the opposite direction, as shown in Fig. 4, F¹.

REGENERATION BY TRANSFORMATION OF THE ENTIRE PIECE

In the regeneration of some of the lower animals, the transformation of a piece into a new animal of smaller size is brought about by a change in form of the piece itself, rather than through the production of new material at the cut-ends. If a ring is cut from the body of hydra, as shown in Fig. 5, A, the open ends of the ring are soon closed by the contraction of the sides of the piece, and in the course of a few hours the ring has become a hollow sphere; or, if the piece is longer, a closed cylinder. After a day or two, the piece begins to elongate, and four tentacles appear near one end (Fig. 5, B, C, D). The piece continues to elongate until it forms a small polyp, having the typical proportions of length to breadth (Fig. 5, E, F). It has changed into a new cylinder that is longer than the piece cut off, but correspondingly narrower. In this case there cannot be said to be a replacement of the missing parts, but rather, through the transformation of the old piece, the formation of a new whole. In planarians also the formation of a new worm from a piece involves a change in the form of the old part, as well as the addition of new material at the cut-end. If a cross-piece is cut out, as shown in Fig. 4, D, new material appears at the ends, but the old piece also becomes narrower and longer (Fig. 4, D¹-D⁴). If the old head is cut off, it produces new material at its posterior end (Fig. 4, E, E¹), and also becomes smaller as the new part grows larger (Fig. 4, E², E³). In a land planarian, Bipalium kewense, a piece is transformed into a new worm, as shown in Fig. 6, A, B. In this case the old pigment stripes of the piece are carried directly over into the new worm, the piece elongating during the transformation.

A similar change takes place in pieces of unicellular animals, as best shown by cutting off pieces of stentor. If Stentor cœruleus is cut in two pieces, as indicated in Fig. 7, each piece makes a new individual of half size, but of proportionate form. The old peristome remains on the anterior piece, but becomes reduced in size as the piece changes its shape, and although it may be at first too large for the length of the new piece, it ultimately reaches a size about proportionate to the rest of the animal. The posterior piece is at first too long for the size of the new peristome that is formed, but the latter becomes larger, until the characteristic form has been reached. The change in form of the stentor may take place in a few hours, and the result is brought about, not by the development of new protoplasm over the cut-end, but by a change of the old protoplasm into the new form. A similar experiment is shown in Fig. 8, in which a stentor was cut into three pieces, each piece containing a part of the old nucleus.

REGENERATION IN PLANTS

In the higher plants the production of a new plant from a piece takes place in a different way from that by which in animals a new individual is formed. The piece does not complete itself at the cut-ends, nor does it change its form into that of a new plant, but the leaf-buds that are present on the piece begin to develop, especially those near the distal end of the piece, as shown in Fig. 32, A, and roots appear near the basal end of the piece. The changes that take place in the piece are different from those taking place in animals, but as the principal difference is the development of the new part near the end, rather than over the end, and as in some cases the new part may even appear in new tissue that covers the end, and, further, since the process seems to include many factors that appear also in animals, we are justified, I think, in including this process in plants under the general term regeneration.

In the lower plants, such as the mosses, the liverworts, the moulds, and the unicellular forms, regeneration also takes place. Vöchting has shown that pieces from any part of the thallus of a liverwort[8] produce new plants. If a cross-piece is cut off, there appears a small outgrowth from the middle of the anterior cut-edge, as shown in Fig. 9, A, A², that gradually enlarges to form a new thallus. It will be seen from the figures that the whole anterior edge does not grow forward, but a new thallus arises from a group of cells at, or near, the anterior edge. These cells are the least-differentiated cells in the piece, and have softer cell walls than have the other cells.

Pringsheim has shown that if a piece of the stalk of the sporangium of certain mosses is cut off, it produces at its ends thread-like outgrowths which are like the protonema-stage of the moss, and from this protonema new moss-plants may arise (Fig. 10, A, B, C, D).

Braefeld has obtained a somewhat similar result in one of the moulds, in which a piece of the sporangium stalk gives rise to a mycelium from which new sporangia may be produced.

REGENERATION IN EMBRYOS AND EGGS

Regeneration takes place not only in adult organisms, but also in embryos, and larvæ of many animals. It is often stated that the power of regeneration is more highly developed in embryos than in adults, but the facts that can be advanced in support of this view are not numerous. One of the few cases of this sort known to us is that of the leg of the frog, that does not regenerate, while the leg of the tadpole is capable of regenerating.

The early stages in the development of the sea-urchin, or of the starfish, may be taken to illustrate the power of regeneration in embryos. If the hollow blastula of the sea-urchin is cut into pieces (Fig. 11, A), each piece, if not too small, may produce a new blastula. The edges of the piece come together, and fuse in the same way in which a piece of hydra closes. A new hollow sphere of small size is formed, which then passes through the later stages of development as does the whole normal blastula.

Still earlier stages of the sea-urchin, or of the starfish, have the power of producing embryos if they are cut into pieces. If the segmenting egg is separated into a few parts, each part will continue to develop. Even the first two blastomeres or cells will, if separated, produce each a whole embryo (Fig. 11, B). The power of development of a part does not even end here, for, if the undivided, fertilized egg is cut into pieces, the part that contains the nucleus will segment and produce a whole embryo (Fig. 11, C, upper row). If the egg is cut in two or more pieces before fertilization, and then each part is fertilized, it has been found that not only the nucleated, but even the non-nucleated fragments (if they are entered by a single spermatozoon) may produce embryos (Fig. 11, C, lower row).

It may be questioned whether the development of parts of the embryo, or of the egg, into a whole organism can be included in the category of regenerative processes. There are, it is true, certain differences between these cases and those of adult forms, but as there are many similarities in the two cases, and as the same factors appear in both, we cannot refuse, I think, to consider all the results from a common point of view.

PHYSIOLOGICAL REGENERATION

Finally, there are certain normal changes that occur in animals and plants that are not the result of injury to the organism, and these have many points in common with the processes of regeneration. They are generally spoken of as processes of physiological regeneration. The annual moulting of the feathers of birds, the periodic loss and growth of the horns of stags, the breaking down of cells in different parts of the body after they have been active for a time, and their replacement by new cells, the loss of the peristome in the protozoon, stentor, and its renewal by a new peristome, are examples of physiological regeneration. This group of phenomena must also be included under the term “regeneration,” since it is not sharply separated from that including those cases of regeneration after injury, or loss of a part, and both processes appear to involve the same factors.

DEFINITION OF TERMS

The older writers used such terms as “replacement of lost parts,” “renewal of organs,” and “regeneration” to designate processes similar to those described in the preceding pages. The term regeneration has been for a long time in general use to include all such phenomena as those referred to, but amongst recent writers there is some diversity of opinion as to how much is to be included in the term, and the question has arisen as to the advantage of applying new names to the different kinds of regeneration. There can be little doubt of the advantage, for the sake of greater clearness, of the use of different terms to designate different phenomena, but I think that there is at the same time the need of some general term to cover the whole field, and the word regeneration, that is already in general use, seems to fulfil this purpose better than any other.

Roux[9] points out that Trembley, and later Nussbaum, showed that a piece of hydra regenerates without the formation of new material. Roux adds that since during development the piece takes no nourishment, the regeneration must be brought about by the rearrangement of the cells present in the piece.[10] The change may, or may not, involve an increase in the number of the cells through a process of division. In consequence of this method of development a re-differentiation of the cells that have been already differentiated takes place. This process of regeneration, Roux points out, is very similar to the “post-generation” of the piece of the blastula of the sea-urchin embryo, and he concludes that “regeneration may be brought about entirely, or very largely, through the rearrangement and re-differentiation of cells without any, or with very little, proliferation taking place.” In the adults of higher animals regeneration by proliferation preponderates, but rearrangement and re-differentiation of cells occur in all processes of regeneration, even in higher vertebrates. The two kinds of regeneration that Roux distinguishes are, he says, essentially quantitative.[11]

Barfurth[12] has defined regeneration as “the replacement of an organized whole from a part of the same.” If the part is given by nature, there is a process of physiological regeneration; if the part is the result of an artificial injury, the process is one of pathological regeneration. Barfurth includes in the latter category the production of a new, entire individual from a piece, as in hydra; regeneration by proliferation, as in the earthworm; and also the development of pieces of an egg or of an embryo.

Barfurth’s definition of regeneration is unsatisfactory, since an egg is itself a portion of an organism that makes a new whole, and this sort of development is not, of course, as he himself points out, to be included in the term regeneration. Nor does the use of the word “replacement” save the definition, since in many cases the kind of part that is lost is not replaced. The use of the word “pathological” to distinguish ordinary regeneration from physiological regeneration is, I think, also unfortunate, since it implies too much. There is nothing necessarily pathological in the process, especially in such cases as hydra, or as in the development of a piece of an egg where the piece is transformed directly into a new organism. Furthermore, in those cases in which (as in some annelids and planarians) a new head is formed after or during the process of natural division, there is little that suggests a pathological process; and in this instance the regeneration takes place in the same way as after artificial section.

Driesch, in his Analytische Theorie, states that Fraisse and Barfurth have established that during regeneration each organ produces only its like. Driesch defines regeneration, therefore, as the re-awakening of those factors that once more bring into play, by means of division and growth, the elementary processes that had ceased to act when the embryonic development was finished. This is regeneration in the restricted sense, but Driesch also points out that this definition must be enlarged, since, when a triton, for example, regenerates its leg, not only does each tissue produce its like, but later a reconstruction and differentiation takes place, so that a leg and foot are formed, and not simply a stump containing all of the typical tissues. Driesch holds that regeneration should include only those cases in which a proliferation of new tissue precedes the development of the new part, and suggests that other terms be used for such cases as those of pieces of hydra, pieces of the egg, etc., in which the change takes place in the old part without proliferation of new tissue. It seems to me unwise to narrow the scope of the word regeneration as Driesch proposes, for it has neither historical usage in its favor, nor can we make any fundamental distinction between cases in which proliferation takes place and those in which it does not. As will be shown later, the factors that are present in the two cases appear to be in large part the same, and while it may be convenient to put into one class those cases in which proliferation precedes the formation of the new organs, and into another class those cases in which the change takes place without proliferation, yet, since the distinction is one of subordinate value, it is necessary to have one word to include both groups of cases; and no better word than regeneration has, I think, been as yet suggested.

Driesch has made use of two other descriptive terms. The word “reparation” is used to describe the development of the hydranth of tubularia. The new hydranth is formed in this case out of the old tissue at the end of the piece (Fig. 20, A). The change appears to be the same as that which takes place in a piece of hydra, etc. The word “reparation” does not seem to me to express very satisfactorily this sort of change, or sharply separate it from those cases in which the animal is repaired by adding what has been taken away; but in this latter sense Driesch does not use the term. I have not made use of the word, in general, except as applied to Driesch’s work.

Another term, “regulation,” used by Roux,[13] and also by Driesch and others, is used in a sort of physiological sense to express the readjustments that take place, by means of which the typical form is realized or maintained. By inference we may extend the use of the word to include the changes that take place in the new material, that is proliferated in forms that regenerate by this method. Driesch uses this term, regulation, to include a much more general class of phenomena than those included in the term regeneration, as for instance, the regulation of metabolism and of adaptation, etc. One of the subdivisions of the term regulation is called “restitution.” This word also is used where I should prefer to use the word regeneration as a general term, and the word reorganization when reference is made to the internal changes that lead to the production of a typical form.

Both Roux and Driesch also speak of “self-regulation,” by which is meant, I suppose, that the changes taking place are due to readjustments in the part itself, and are not induced by outside factors. The expression “self-regulation” is not, I think, a very happy one, since all change is ultimately dependent upon a relation between inside and outside conditions.

Hertwig[14] defines regeneration as the power of replacement of a part of the organism. He states that in all cases the beginning of the process is the same, viz. the appearance of a small protuberance composed of cells, that is the rudiment of the new part. It is evident that Hertwig has taken into account only one side of the process. Those cases in which a rearrangement or reorganization takes place in the old part are not even considered.[15] Goebel[16] points out that in plants the fully formed cells are, as a rule, incapable of further growth after they have once served as a basis of an organ of the body, but often some of the cells may remain in a latent condition, and grow again, when the intercellular interactions are disturbed. This is the case, he thinks, in regeneration. Goebel speaks of regeneration by means of adventitious buds in those cases in which the buds had not previously existed before the removal of the part. In those cases in which the buds are in existence before the piece is removed, as in the leaves of Asplenium, Begonia, etc., the development is not the result of regeneration, Goebel thinks, but the buds represent a stage in the development of the species. It may be pointed out, however, that it is certainly a remarkable fact that often the conditions that lead to the unfolding of an existing bud are the same as those that lead to the development of a new bud.

The preceding account will suffice to illustrate some of the principal ideas that are held in regard to the process of regeneration. Since many new facts have come to light in the last few years, it may not be amiss to point out what terms will be used in the following pages to include each kind of process.

The word “regeneration” has come to mean, in general usage, not only the replacement of a lost part, but also the development of a new, whole organism, or even a part of an organism, from a piece of an adult, or of an embryo, or of an egg. We must include also those cases in which the part replaced is less than the part removed, or even different in kind.

At present there are known two general ways in which regeneration may take place, although the two processes are not sharply separated, and may even appear combined in the same form. In order to distinguish broadly these two modes I propose to call those cases of regeneration in which a proliferation of material precedes the development of the new part, “epimorphosis.” The other mode, in which a part is transformed directly into a new organism, or part of an organism without proliferation at the cut-surfaces, “morphallaxis.”

In regard to the form of the new part, certain terms may be used that will enable us to characterize briefly different classes. When the new part is like that removed, or like a part of that removed, as when a leg or a tail is regenerated in a newt, the process is one of

“homomorphosis.”[17] Under this heading we may distinguish two cases, in one of which the entire lost part is at once, or later, replaced—holomorphosis; in the other the new part is less than the part removed—meromorphosis. When the new part is different from the part removed the process has been called by Loeb “heteromorphosis,” but there are at least two different kinds of processes that are covered by this definition. In one case the new part is not only different from the part removed, but is also an organ that belongs to a different part of the body (or it may be unlike any organ of the body). This we may call “neomorphosis.” As an illustration of this process may be cited the development of an antenna, when the eye of a crab or of a prawn is cut off near the base (Fig. 12); and as an example of an organ different in kind from any organ of the same animal, may be cited the case of Atyoïda potimirum, in which the new leg is unlike any other leg on the body. The name “heteromorphosis” can be retained for those cases in which the new part is the mirror figure of the part from which it arises, or more generally stated, where the new part has its axes reversed as compared with the old part. As an example of this may be cited the development of an aboral head on the posterior end of a piece of the stem of Tubularia (Fig. 15, B), or the development of a tail at the anterior end of a posterior piece of an earthworm (Fig. 2).

The term “physiological regeneration” I shall use in the ordinary sense to include such changes as the moulting and replacement of the feathers of birds, the replacement of teeth, etc.,—changes that are a part of the life-cycle of the individual. In some cases it can be shown that these processes are closely related to ordinary regeneration, as when a feather pulled out is formed anew without waiting for the next moulting period, and formed presumably out of the same rudiment that would have made the new feather in the ordinary moulting process.

It is sometimes convenient to contrast the process of physiological regeneration with all other kinds. The use of the term “pathological regeneration” for the latter seems to me, as has been said, unsatisfactory. The two terms proposed by Delage,[18] viz. “regular regeneration” and “accidental regeneration,” have certain advantages, although there is nothing accidental, or at least occasional, in regard to the process itself, as it is entirely regular, although it may only occur after an accident to the animal. The term “regular regeneration” is, I think, more satisfactory than “physiological regeneration,” but the latter has the advantage that it has come into current use. For what is known as pathological or accidental regeneration, I propose the term “restorative regeneration,” and I shall continue to use the term “physiological regeneration” as generally understood.

CHAPTER II

THE EXTERNAL FACTORS OF REGENERATION IN ANIMALS

In general it may be stated that the limits of temperature under which normal growth may take place represent also the limits of temperature for regeneration. Lillie and Knowlton (’97) have determined the limits of temperature within which regeneration takes place in Planaria torva. The worm was cut in two transversely through the pharynx, and the time required at different temperatures to produce a new head on the posterior piece was recorded. The lowest temperature at which regeneration was found to take place was 3°C. Of six individuals kept at this temperature only one regenerated at all, and in this one the eyes and brain were still incomplete after six months. The optimum temperature, or at least that at which regeneration takes place most rapidly, was found to be 29.7°C.; a new head developed in 46 days at this temperature. At 31.5°C. regeneration was slower, requiring 8.5 days to make a new head. At 32°C. incomplete regeneration sometimes took place, but death occurred in about six days. At 33°C. regeneration was very slight, and the animals died within three days. At 34°C., and above this point, no regeneration took place, and death soon occurred.

In Hydra viridis, Peebles (’98) has found that regeneration is quicker at 26°-27°C. than at 28°-30°C. At the former temperature regeneration takes place in 48 hours. If kept at 12°C. pieces may regenerate in 96 hours, but not all the pieces had regenerated in this case until 168 hours.

INFLUENCE OF FOOD ON REGENERATION

While the growth of an animal or of a plant is, in most cases, and, of course, within certain limits, directly connected with the amount of food that is obtainable, nevertheless extensive regeneration may take place in an animal, or part of an animal, entirely deprived of food. In this case the material for the new part is derived from the excess of material in the old part, and not only surplus food material, but even the protoplasm itself appears to be drawn upon to furnish material to the new part. The relation between regeneration and the amount of food present in the old part is well shown by experiments with planarians. If a planarian is kept for several months without food, it will decrease very much in size. In fact, the volume of a starved worm of Planaria lugubris compared with that of a fully fed individual may be only one-thirteenth of the latter (Fig. 13, A, B). If a starved worm is cut in two pieces, each piece will regenerate, although less quickly than in a well-fed worm. The new part will continue to increase in size at the expense of the old piece that is already in a starved condition. On the other hand, an excess of food does not necessarily produce a hastening of the regeneration, for, as Bardeen (’01) has shown, worms that have been for several days without food may regenerate more quickly than worms that have been fed just before they were cut into pieces.

The growth of the new part at the expense of the old tissues is a phenomenon of the greatest importance, an explanation of which will involve, I think, the most fundamental questions pertaining to

growth. The results show that growth is connected with a structural factor, and is not simply a physiological phenomenon, although no doubt physiological factors are involved. But the physiological factors that are here at work seem to be different from what is ordinarily understood; for the fact that a tissue that is slowly starving to death should be reduced still further, and at a more rapid rate, in order to supply material to a new part, is certainly a remarkable phenomenon. At present we are not in a position to offer any explanation that rests on observation, or experiment, as to how the transfer of material takes place, or as to how the new tissue manages to get hold of the material from other parts. It is possible to protect the old part to a large extent by keeping the regenerating piece well supplied with food. If a well-fed planarian is cut in two along the middle line of the body as indicated in Fig. 13½, A, there develops, in the course of five or six days after the operation, new material along the cut-side of each piece, and a new pharynx appears at the border between the old and the new parts. If one of the pieces is fed at intervals, it is found that the new part grows more rapidly than does the new part in the piece without food. The old tissue in both pieces has shortened somewhat after the operation, and has also decreased somewhat in size as the first new material developed along the cut-side, but in the piece that is fed the old half begins to increase again until it reaches its former size, and may even surpass the latter. A large full-sized worm is produced from this piece, as shown in Fig. 13½, B, C, D. In the starved piece the old part continues to grow small, due to the lack of food and also to the increase in the new side. This increase takes place very slowly, but ultimately a small symmetrical worm may be produced, as shown in Fig. 13½, E, F, G. It will be seen that the starved piece needs to produce relatively less and less new material in order to become symmetrical, because as the old material diminishes, the pharynx comes to lie nearer to the middle line.

EFFECT OF LIGHT ON REGENERATION

Although few experiments have been made to test the effect of light on regeneration, it is certain that in many cases light has no effect on the process, neither as to the quality nor the quantity of the result. In one form, a tubularian hydroid, Eudendrium racemosum, it has been shown by Loeb that the regeneration of the hydranth takes place only when the animal is exposed to light. When a colony of eudendrium is brought into the laboratory and placed in an aquarium, the hydranths soon die; but if the colony is kept in a lighted aquarium, new hydranths are regenerated in a few days. If, on the other hand, the colony is kept in the dark, new hydranths do not appear; but if it is brought back again into the light the hydranths appear. In one experiment one lot of pieces was kept in diffuse daylight, and another lot in the dark. The former produced fifty new hydranths in a few days; those in the dark had not made any hydranths after seventeen days. They were then brought into the light, and in a few days several hydranths had developed on each piece.

Loeb also tried the effect of different colored light on the regeneration of eudendrium. Dishes containing pieces of the hydroid were put into a box that was covered by colored glass plates. Pieces subjected to dark red and to dark blue light gave the following results. The old hydranths, as is generally the case, were absorbed in the course of three days. The first new hydranths appeared in the blue light on the fourth day, and during the following days the hydranths in this lot steadily increased. Eight days after the beginning of the experiment there were eighty hydranths under the blue glass, but not one had developed in the red light. On the ninth day the red glass was replaced by a dark blue one. Two days later hydranths began to appear, and on the following day thirty-two hydranths had appeared, and in a few days more as many as sixty had developed.[19] Loeb concluded that only in the more refrangible (blue) rays does the regeneration of the hydranth take place, while the less refrangible (red) rays act as darkness does.[20] This hydroid is the only animal yet found that shows the effect of light on regeneration, and it is interesting to find that it is one of the few animals known in which light has an influence on the growth, if the heliotropism, or turning towards the light, of the hydranth is looked upon as a phenomenon of growth.

There is another series of experiments made to test the effect of light on regeneration, which gave, however, negative results. Herbst observed that when the eye of certain crustacea[21] is cut off, sometimes an eye and sometimes an antenna is regenerated. A number of individuals from which the eyes had been removed were kept in the light, and others in the dark, in order to see if the presence or absence of light is a factor in determining the kind of regeneration that takes place. It was found that as many individuals regenerated eyes in the dark as in the light. It was discovered later by Herbst and myself, independently, that, when the end only of the eye-stalk is cut off, an eye regenerates, but when the eye-stalk is cut off at the base, an antenna regenerates. The difference in the result has therefore no connection with the presence or absence of light.

GRAVITY

The only case known amongst animals, in which regeneration is influenced by the action of gravity,[22] is that of the hydroid Antennularia antennina. This hydroid lives attached to the bottom of the sea several metres below the surface. The hydroid consists of a single, vertical, central stem, or axis, with two or four series of lateral branches along which the hydranths arise (Fig. 14, A). The stem is attached by so-called stolons, or roots. In its normal growth at the free end the hydroid has been shown by Loeb to exhibit marked geotropic changes. If, for instance, the stem is bent over to one side the new growth that takes place at the apex of the stem directs the new part upwards in a vertical direction.

If pieces are cut from the stem of antennularia and suspended in