Regeneration

As an example of looseness of expression I may quote the following from Weismann : "A useless or almost useless rudimentary part may often be injured or torn off without causing processes of selection to occur iv/iich would produce in it a capacity for regeneration. The tail of a lizard again, which is very liable to injury, becomes regenerated because, as we have seen, it is of great importance to the individual and if lost its owner is placed at a disadvantage." And as an example of vagueness, the following statement commends itself : " Finally the complexity of the individual parts constitutes the third factor which is concerned in regulating the regenerative power of the part in question ; for the more complex the structure is, the longer and the more energetically the process of selection must act in order to provide the mechanism of regeneration, which consists in the equipment of a large number of different kinds of cells with the supplementary determinants which are accurately graduated and regulated as regards their power of multiplication."

Without attempting to disentangle the ideas that are involved in these sentences, let us rather attempt to get a general conception of Weismann's views. In a later paper (1900), in reply to certain criticisms, he has stated his position somewhat more lucidly. In the following statement I have tried to give the essential part of his hypotheses : Weismann believes the process of regeneration to be regulated by " natural selection " ; in fact, he states that it has arisen through such a process in the lower animals — since they are more subject to injury — and that it has been lost in the higher forms except where, on account of injury, it has been retained in certain parts. Thus when Weismann speaks of regeneration as being an adaptation of the organism to its environment, we must understand him to mean that this adaptation is the result of the action of natural selection. We should be on our guard not to be misled by the statement that because regeneration is useful to the animal, it has been acquired by natural selection, since it is possible that regeneration might be more or less useful without in any way involving the idea that natural selection is the originator of this or of any other adaptation. It will be seen, therefore, that in order to meet Weismann on his own ground it will be necessary to have a clear understanding in regard to the relation of regeneration to Darwin's principle of natural selection. With Weismann's special hypothesis of the " mechanism," so-called, by which regeneration is made possible we have here nothing to do, but may consider it on its own merits in another chapter.

In order to have before us the material for a discussion of the possible influence of natural selection on regeneration, let us first examine the facts that bear on the question of the liability of the parts to injury and their power to regenerate, and in this connection the questions concerning the renewal of parts that are thrown off by the animals themselves in response to an external stimulus are worthy of careful consideration. A comparison between the regeneration of these parts with that of other parts of the same animal gives also

important data. Furthermore, a comparison may be made between different parts of the same animal, or between the same parts of different animals living under similar or dissimilar conditions. There are only a few cases known in which a systematic exami- •nation has been carried out of the power of regeneration of the different parts of the body of the same animal. Spallanzani's results show that those salamanders that can regenerate their fore legs can regenerate their hind legs also. Towle, who has examined in my laboratory the regeneration of a number of American newts and salamanders, finds also that both the fore and hind legs regenerate in the same forms. The tail and the external gills, in those newts with gills, also regenerate. It has also been shown in triton that the eye regenerates if a portion of the bulb is left. Broussonet first showed (1786) that the fins of fish have the power to regenerate, although, strangely enough, Fraisse and Weismann state that very little power of regeneration is present in the fins of fish. I have found that the fins of several kinds of fish regenerate, belonging to widely different families.1 In Fnndnlns heteroclitus I have found that the pectoral, pelvic, caudal, anal, and dorsal fins have the power of regeneration. In reptiles the feet do not regenerate, — at least no cases are known, — but the tail of lizards has this power well developed. In birds neither the wings nor the feet regenerate, but Fraisse has described the case of a stork in which, the lower jaw being broken off, and the upper being cut off at the same level, both regenerated. Bordage has recorded the regeneration of the beak of the domesticated fighting cocks (of the Malay breed) of Mauritius. In the mammals neither the legs, nor the tail, nor the jaws regenerate, although several of the internal organs, as described in the next chapter, have extensive powers of regeneration.

The best opportunity to examine the regenerative power in similar organs of the same animal is found in forms like the Crustacea, myriapods, and insects, in which external appendages are repeated in each or many segments of the body. In decapod Crustacea, including shrimps, lobsters, crayfish, crabs, hermit-crabs, etc., regeneration takes place in the walking legs of all the forms that have been examined, and this includes members of many genera and families. I have made an examination of the regeneration of the appendages (Fig. 37) of the hermit-crab. In this animal, which lives in an appropriated snail's shell, only the anterior part of the body projects from the shell. The part that protrudes is covered by a hard cuticle, while the part of the body covered by the shell is quite soft. Three pairs of legs are protruded from the shell. The first pair with large claws

1 Fundulus heteroclitus, Stenopus chrysops, Decapterus macrella, l\fenticirrhus macre/la, Carassins miralus, Phoxinus fundtdoides, Noturus s/>., arrl a few others. are used for procuring food, and as organs of offence and defence ; the second and third pairs are used for walking. The following two pairs, that correspond to the last two pairs of walking legs of crabs and crayfishes, are small, and are used by the animal in bracing itself against the shell. The first three pairs of legs have an arrange-

FlG. 37. — Appendages of Hermit-crab (Eupagurus longicarpus} . A. Third walking leg. B. Next to last thoracic leg. /?'. Last thoracic leg. C, Cl, C2. Three abdominal appendages of male. D. Telson and sixth segment with last pair of abdominal appendages. E. Regeneration of new leg from cut-end outside of " breaking-joint." F. Leg regenerating from cut made inside of " breaking-joint." G. Leg regenerating from cut made very near the body. ment at the base, the " breaking-joint," by means of which the leg is thrown off, if injured. The last two pairs of thoracic legs cannot be thrown off. The first three pairs of legs are often lost under natural conditions. In an examination of 188 individuals I found that 21 (or 1 1 per cent) had lost one or more legs. If one of the first three legs is injured, except in the outer segment, it is thrown off at the break-

ing-joint, and a new leg regenerates from the broken-off end of the stump that is left. The new leg does not become full size, and is of little use until the crab has moulted at least once. The leg breaks off so close to the body, and the part inside of the breaking-joint is so well protected by the bases of the other legs, that it is scarcely possible that the leg could be torn off inside of the breaking-joint, and, as a matter of observation, all crabs that are found regenerating their legs under natural conditions do so from the breaking-joint. If, however, by means of small scissors, the leg is cut off quite near the body, a new leg regenerates from the cut-end, even when the leg is cut off at its very base. The breaking-joint would thoroughly protect from injury the part of the leg that lies nearer to the body, and yet from this inner part a new leg is regenerated. Moreover, the new leg is perfect in every respect, even to the formation of a new breaking-joint. In this case we have a demonstration that there need be no connection between the liability of a part to injury and its power of regeneration.

In still another way the same thing may be shown. If the crab is anaesthetized, and a leg cut off outside of the breaking-joint, it is not, at the time, thrown off — the nervous system, through whose action the breaking off takes place, being temporarily thrown out of order. After recovery, although the leg is thrown off in a large number of cases, it is sometimes retained. In such cases it is found that from the cut-end the missing part is regenerated. In this case also we find that regeneration takes place from a part of the leg that can never regenerate under natural circumstances.

The third and fourth legs of the hermit-crab cannot be thrown off, but they have the power of regeneration at any level at which they may be cut off. They are in a position where they can seldom be injured, and I have never found them absent or injured in crabs caught in their natural environment. The soft abdomen is protected by the snail's shell. At the end .of the abdomen the last pair of abdominal appendages serve as anchors to hold the crab in the shell. These appendages are large and very hard, and can seldom be injured unless the abdomen itself is broken, and under these circumstances the crab dies. Yet if these appendages are cut off they regenerate perfectly, and after a single moult cannot be distinguished from normal ones.

The more anterior abdominal appendages are present only on one side of the adult, although they are present on both sides of the larva, and, to judge from a comparison with other Crustacea, these appendages have degenerated completely on one side, and have become rudimentary in the male, even on the side on which they are present. They too will regenerate if they are cut off. In the female these appendages are used to carry the eggs, and are, therefore, of use. They also have a similar power of regeneration. The maxillae and maxillipeds of the hermit-crab have likewise the power of regeneration, as have also the two pairs of antennas and the eyes.

In other decapod Crustacea also it has been shown that the power of regeneration of the appendages is well developed. It has been long known that the crayfish and the lobster can regenerate lost parts. The first pair of legs, or chelae, in these forms has a breakingjoint, at which the leg can be thrown off, yet in the crayfish I have seen that if the leg is cut off inside of the breaking-joint it will regenerate. The four pairs of walking legs do not possess a breaking-joint, but may be thrown off in some cases at a corresponding level. They regenerate from this level, as well as nearer the body and farther beyond this region. Przibram has recently shown that, in a number of Crustacea, regeneration of the appendages takes place, even when the entire leg is extirpated as completely as possible.

Newport has shown that the myriapods can regenerate their legs, and it is known that several forms have the power of breaking off their legs in a definite region at the base if the legs are injured, and I have observed in Cermatia forceps that this takes place even when the animal is thrown into a killing fluid. Newport ('44) has also shown that when the legs of a caterpillar are cut off new ones regenerate during the pupa stage. It has been long known 1 that the legs of mantis can regenerate, and Bordage, who has recently examined the question more fully, has shown that a breaking-joint is present at the base of the leg. The tarsus of the cockroach also regenerates, producing only four, instead of the five, characteristic segments.2

A number of writers have recorded the regeneration of the legs of spiders.3 Schultz, who has recently examined more thoroughly the regeneration of the legs in some spiders, finds that the leg is renewed if cut off at any level. He removed the leg most often at the metatarsus, but also at the tibia, and generally between two joints. In some cases the leg was cut off at the coxa, at which level it is generally found to be lost under natural conditions. Wagner observed in tarantula that when the leg. is removed at any other place than at the coxa, the animal brings the wounded leg to its jaws, and bites it off down to the coxa. In the Epcitida, that Schultz chiefly made use of, this never happened. He observed, however, even in these forms, that when the leg is cut off at the coxa it regenerates better than

when cut off at any other level. Schultz states that we see here an excellent example of how regeneration is influenced by natural selection, since regeneration takes place best where the leg is most often broken off. On the other hand, the author hastens to add that since regeneration also takes place when the leg is cut off at any other FIG. 38. — A-F. After King. A. Starfish with four arms regenerating at different levels. B. Three arms regenerating from disk. C. Arm split in two producing two arms. D. Arm cut off obliquely, regenerating at right angles to cut-surface. E. Starfish split between two aims, producing two new arms from split. F. An arm, with a small piece of disk attached, regenerating three new arms. G. After P. and F. Sarasin. Starfish (Linckia multiformis) with four new arms springing from end of one arm. Interpreted as a new starfish, but probably only multiple arms (see C, above).

level, this shows that the power to regenerate is characteristic of all parts of the organism, and is not merely a phenomenon of adaptation, as Weismann believes. It seems highly improbable that a spider could ever lose a leg in the middle of a segment, i.e. between two joints, since the segments are hard and strong and the joints much weaker; but nevertheless the leg has the power to regenerate also from the middle of the segment, if cut off in this region.

The formation of the new part takes place somewhat differently, according to Schultz, when the leg is amputated between two segments than when cut off at the coxa. In the latter case, there is produced from the cut-end of the last segment a solid rod which, as it grows longer, bends on itself several times. Joints appear in the rod, beginning at the base. The leg is set free at the next moult. If the leg is cut off nearer the distal end a smaller rod is formed, that extends straight forward, or may be thrown into a series of folds. It lies, however, inside of the last segment, since the surface exposed by the cut is quickly covered over by a chitinous covering. The piece is set free at the next moult.

Loeb has found that if the body of the pycnogonid, Pkoxichilidium maxillare, is cut in two there regenerates from the posterior end of the anterior half a new body-like outgrowth. Without attempting to describe the many cases in worms and mollusks in which there is no obvious connection between the power of the part to regenerate and its liability to injury, but where it is more difficult to show that it may not exist, let us pass to an examination of the regeneration of the starfish. It has been known since the time of Reaumur that starfish have the power of regenerating new arms if the old ones are lost. It has been stated that in certain starfishes an arm itself can produce a new starfish, — Haeckel ('78), P. and R. Sarasin ('88), von Martens ('84), and Sars ('75, — but this has been denied by other observers. In several species of starfishes, the separated arm does not regenerate ; but if a portion, even a small piece, of the disk is left with the arm, a new disk and arms may develop (Fig. 38, F). When the arm of Asterias vnlgaris is injured it pinches off in many cases at its base, and a new arm grows out from the short stump that remains. When these starfishes regenerate new arms in their natural environment, the new arms almost always arise from this breaking region.1 Thus King found out of 1914 individuals of Asterias vulgaris collected at random, 206, or 10.7 per cent, had one or more new arms, and all these except one arose from near the disk. In other species it appears that the outer portions of the arm may be broken off without the rest of the arm being

1 The Sarasins have described several cases in Linckia multiformis in which an old arm has one or more new arms arising from it. In one case (copied in our Fig. 38, G"), four rays arise from the end of one arm, producing the appearance of a new starfish. In fact the Sarasins interpret the result in this way, although they state that there is no madreporite on the upper surface, and they did not determine whether a mouth is formed at the convergence of the rays, because they did not wish to destroy so unique a specimen — even to find out the meaning of it. There seems to me little probability that the new structure is a starfish, but the. old arm has been so injured that it has produced a number of new arms.

thrown off. King has found that in asterias, regeneration takes place more rapidly from the base than at a more distal level. It may appear, at first thought, that the more rapid regeneration of the arm at the place at which it is usually thrown off may be associated with its more frequent loss at this region — in other words, that the more rapid regeneration has been acquired by the region at which the arm is generally broken off. This interpretation is, however, excluded by the fact that, in general, the nearer to the base the arm is cut off, so much the more rapid is its regeneration. In other words, the more rapid regeneration of the arm at the base is only a part of a general law that holds throughout the arm. If the proposition is reversed, and it is claimed that the arm has acquired the property of breaking off at the base, because it regenerates more rapidly at that level, the following fact recorded by King is of importance, viz. that, although the arm regenerates faster at the base, yet a new arm is not any sooner produced in this way, since there is more to be produced and the new arm from the base may never catch up to one growing less rapidly from a more distal cut-surface, but having a nearer goal to reach.

The results of our examination show that those forms that are liable to have certain parts of their bodies injured are able to regenerate not only these parts, but at the same time other parts of the body that are not subject to injury. The most remarkable instance of this sort is found in those animals having breaking-joints. In these forms, we find that regeneration takes place both proximal and distal to this region. If the power of regeneration is connected with the liability of a part to injury, this fact is inexplicable.

Turning now to the question as to whether regeneration takes place in those species that are subject to injury more frequently or better than in other species, we find that the data are not very complete or satisfactory for such an examination. It is not easy to ascertain to what extent different animals are exposed to injury. If we pass in review the main groups of the animal kingdom, we can at least glean some interesting facts in this connection.

In the protozoa nucleated pieces have been found to regenerate in all forms that have been examined, including amoeba, difflugia, thalassicolla, paramcecium, stentor, and a number of other ciliate infusoria. In the sponges it has been found by Oscar Schmidt that pieces may produce new individuals, but how widely this occurs in the group is not known. In the ccelenterates many forms are known to regenerate, and it is not improbable that in one way or another the process occurs throughout the group. The hydroid forms, hydra, tubularia, parypha, eudendrium, antennularia, hydractinia, podocoryne, etc.,

the jelly-fish, gonionemus, and certain members of the family TJiaumantida, have been found to regenerate. Amongst the Scyp/iozoa, metridium, cerianthus, and the scyphistoma of aurelia regenerate, and the jelly-fishes belonging to this group have a limited amount of regenerative power. In the platodes we find that all the triclads, thus far examined, including planaria, phagocata, dendrocoelum, and the land triclad, bipalium, regenerate. It has been shown that the marine triclads also regenerate, but less rapidly and extensively, while the marine polyclads have very limited powers of regeneration. The regeneration of the trematodes and cestodes has not, so far as I know, been studied, neither have the nematodes been examined from this point of view.

Some of the nemerteans regenerate, others do not seem to have this power. A small fresh-water form, tetrastemma, that I examined, did not regenerate, although some of the pieces, that were filled with eggs, remained alive for several months. In the annelids we find a great many forms that regenerate — many marine polychaeta have this power; all oligochaeta that have been studied regenerate ; both land forms, like lumbricus, allolobophora, etc., and fresh-water forms, like lumbriculus, nais, tubifex, etc.

In the Crustacea the appendages have the power to regenerate in all the forms that have been examined. Several kinds of myriapods, as well as a number of spiders, are known to regenerate their legs. In the insects, however, only a few forms are known to have this power, — caterpillars, mantis, and the cockroach. The large majority of insects, in the imago state, do not seem to be able to regenerate, although in a few cases regeneration has been found to occur.1

In the mollusks, regeneration of the head takes place under certain conditions. Spallanzani thought that if the entire head is cut off a new one regenerates. This conclusion was denied by at least eleven of his contemporaries, and confirmed by about ten others. It was found later that the result depends in part on the time of year and in part on the kind of snail. Carriere, who more recently examined the question, found that even under the most favorable conditions regeneration does not take place if the circumoesophageal nerve-commissure is completely removed with the head, but if a part remains, a new head develops. It has been stated that a new foot regenerates in helicarion, and I have found that the foot regenerates also in the freshwater snails, physa, limnsea, and planorbis. If the margin of the shell of a lamellibranch or of a snail is broken off, it is renewed by the mantle. The arms of some of the cephalopods are known to regenerate, particularly the hectocotylized arm.

In all the main, groups of echinoderms, with one possible exception, regeneration has been found to take place. Probably all starfishes and brittle-stars regenerate their arms, and even if cut in two or more pieces, new starfishes develop. The crinoids regenerate lost arms, and even parts of the disk ; also the visceral mass. The holothurians have very remarkable powers of regeneration. In some forms regeneration takes place if the animals are cut in two, or even in more than two pieces. The remarkable phenomenon of evisceration that take place in certain holothurians, if they are roughly handled, or kept under unfavorable conditions, are well known and have been described by a number of writers. It has even been suggested that the holothurian may save itself by offering up its viscera

FlG. 39. — A. Amphiuma means with left fore and hind leg regenerating. B. Necturvs maculatus with right fore leg beginning to regenerate after eight months. C. Plethedon cinereus. A, B, C. Drawn to same scale. to its assailant ! Unfortunately for this view, it has been found that the viscera are unpalatable, at least to sea-anemones and to fishes. Ludwig and Minchin suggest that the throwing off of the Cuvierian organs, which are attached to the cloaca, is a defensive act, and if carried too far, according to the latter writer, the viscera may also be lost. The holothurians have remarkable recuperative powers and may regenerate new viscera in a very short time. The sea-urchins form, perhaps, an exception in this group, since there are no records of their regenerative power, but no doubt this is because they have not been as fully investigated as have other forms.

In the vertebrates the lower forms, amphioxus, petromyzon, and sharks, have not been studied in regard to their regenerative power. In the teleostean fishes the fins of a number of forms are known to regenerate. It is probable that this takes place in most members of the group. In the amphibia we find a large number of forms that regenerate their limbs and tail, and other parts of the body, but limitations appear in certain forms. The rapid regeneration of the legs in the smaller urodeles has been often described. In larger forms it takes place more slowly, at least in large forms having large legs. In proteus the regeneration may extend over a year and a half, and in necturus it takes more than a year to make a new limb, at least in animals in confinement. In the large form, amphiuma, that has extremely small legs, regeneration takes place much more rapidly than in a form like "necturus having much larger legs (Fig. 39).