Regeneration

by Thomas Hunt Morgan

A study of the forms that propagate by means of self-division shows that the process is present in many groups of the animal kingdom. In the unicellular forms this method is universally present; and in the multicellular forms the division of the individual cells is looked upon as a process similar to the method of propagation in the protozoa. The sponges do not multiply by self-division. In the cœlenterates, on the other hand, we find this mode of propagation present in most forms. Hydra appears rarely, if at all, to divide by a cross-division, and, although one or two cases of longitudinal division have been described, it is not improbable that they have been started by the accidental splitting of the oral end. The hydromedusæ, Stomobrachium mirabile, Phialidium variabile, Gastroblasta Raffælei, are known to increase by division.[66] Several actinians and many corals divide longitudinally, while the scyphistoma of the scyphomedusæ produce free-swimming ephyras by cross-divisions of the fixed strobila stage. The ctenophors do not divide.

It is known that several fresh-water planarians propagate by division, the tail-end breaking off in the region behind the old pharynx. In one form,[67] and possibly in others, regeneration may begin before the separation takes place. Many of the rhabdocœlous planarians increase by cross-division—the separation taking place more nearly in the middle of the body. In these forms the parts develop new organs more or less completely before they separate. In the trematodes self-division does not take place. The division of the body of the tapeworm into proglottids may represent a process of self-division, but the proglottids do not regenerate after separation.

The nemertians break up readily into pieces, if roughly treated or if the conditions of life are unfavorable, but this can scarcely be spoken of as a process of voluntary self-division. Regeneration takes place in some species, but imperfectly or not at all in others.

In the group of annelids we find many cases of self-division, especially in marine polychætes and in fresh-water oligochætes. One of the most interesting forms, belonging to the first group, is the palolo worm in which the swimming headless form, that is set free by division, serves to distribute the sexual products. Subsequently it appears that the piece dies without regenerating a new head. If we examine more in detail some of the cases of self-division in annelids, we find the following interesting facts. In nereis the posterior region of the body undergoes great changes of structure, the new worm being known under a different name, viz. heteronereis. In this part of the worm, eggs (or sperm) are produced, but it does not separate from the anterior end as a distinct individual. In the family of scyllids the changes that take place in the posterior or sexual end of the body are often accompanied by non-sexual modes of fission. In some species the changes that take place are like those in nereis, and no separation occurs; in other species the sexual region becomes separated from the anterior or non-sexual regions. In scyllis a new head develops, after separation, on the sexual or posterior piece. A new tail is also regenerated by the non-sexual or anterior piece, and as many new segments are formed as are lost. The new posterior region may again produce sexual cells, and again separate. In autolytus a new head develops on the posterior piece before it separates. A region of proliferation is also found at the posterior end of the anterior part. In some species new individuals develop in this zone of proliferation, and a chain of as many as sixteen worms may be present before the one first formed drops off. A still more complicated process is found in myriana. The region just in front of the anus elongates, and gives rise to a large number of segments. These form a new individual with the head at the anterior end. Then another series of segments is proliferated at the posterior end of the old, or anterior worm, and just in front of the first-formed individual. This region also makes a new individual. The process continuing, a chain of individuals is produced, with the oldest individual at the posterior end and the youngest at the anterior end of the series. Each individual grows larger, and produces more segments at its posterior end. Reproductive organs appear in each individual, and when the germ-cells are mature the chain breaks up.

None of the earthworms propagate by self-division, although occasionally, under unfavorable conditions, pieces may pinch off at the posterior end.[68] Lumbriculus, on the other hand, propagates by self-division, although it has been disputed whether the division takes place without the intervention of an external injury or disturbance of some sort, or whether the division may take place entirely from internal causes, that is, spontaneously. Von Wagner has shown that at certain seasons lumbriculus breaks up much more readily than at other times, which may only mean that it is more sensitive to stimuli at one time than at another.

The pieces into which lumbriculus breaks up regenerate after separation. In another form, Ctenodrilus monostylos, division takes place first in the middle of the body behind a cross-septum. Each half may again divide in the same way, and the same process may be repeated again and again until some of the pieces are reduced to a single segment. A new anterior and posterior end may then develop on each piece. In Ctenodrilus pardalis each segment of the middle region of the body constricts from the one in front and from the one behind, and each produces a new head at its anterior end and an anal opening at its posterior end. The worm then breaks up into a number of separate worms. In this series, self-division of the individual is not associated with the development of sexual forms, but seems to be a purely non-sexual method of reproduction. In the leeches self-division does not occur, and no cases are known in the mollusks.

In the echinoderms several forms reproduce by voluntary self-division. In the brittle-stars some forms divide by the disk separating into two parts, one having two and the other three of the old arms. Each piece of the disk then regenerates the missing part of the disk as well as the additional arms. In the starfishes the arms may be thrown off if injured, and, while in certain forms the lost arm does not regenerate a new disk, yet, according to several writers, it may in other species regenerate a new animal. Dalyell observed a process of self-division in a holothurian, each part producing a new individual, and more recent observers have confirmed this discovery.

No cases of self-division are known in the groups of myriapods, insects, crustaceans, spiders, polyzoans, brachiopods, enteropneusta, or vertebrates.

Before discussing the general problems connected with the preceding cases, I should like to point out that it is certainly a striking fact that in all, or nearly all, of these cases of self-division, the separation takes place in the shortest axis, without regard to the structure of the animal. A law similar to that enunciated in connection with the division of the cell seems to hold for the organism as a whole: namely, division takes place, as a rule, in the shortest diameter of the form. The protozoa are, in a sense, excluded, since being unicellular forms they come under the rule for the division of the cell. In the cœlenterates we find the actinians and corals, that have short, cylindrical bodies, dividing from the oral to the aboral end, while the longer scyphistoma divides transversely. The flat, bell-shaped medusa, gastroblasta, divides in an oral-aboral plane. The flat-worms and annelids divide transversely, and, therefore, in the plane of least resistance. The most important illustrations of this principle are furnished by the echinoderms. Those brittle-stars that divide through the disk do so in the shortest direction, that is, from the oral to the aboral side, whilst the holothurians that are long, cylindrical forms divide across the body and, therefore, in a structural plane at right angles to that of the brittle-stars. It may be claimed that in all these cases the plane of division is that in which the animal is most likely to be broken in two by external agents, but this is, I think, only a coincidence, and the result is really due to internal conditions. The division is brought about in most cases, and perhaps in all, by the contraction of the muscles; and the arrangement of the muscles in connection with the form of the body is the real cause of the phenomenon.

Returning to the general question of the occurrence of the process of division in the different groups, we find that in nearly all of them in which self-division occurs it is found in a number of different forms in the same group. The process seems to be characteristic of whole groups rather than of species, and so far as evidence of this sort has any value it points to the conclusion that the process is not necessarily a special case of adaptation to the surroundings, because the species that divide may live under very diverse conditions.