INFLUENCE OF NERVE-CENTERS ON RATE OF REGENERATION.
Previous studies on the influence of the nervous system upon re generation have given very divergent results which can hardly be recon ciled even when we consider that widely separated groups of animals were used as the material for experimentation.
While certain students of this problem (Herbst, Goldstein, Walter, Wolff) have taken the position that the nervous system in general, or some portion of it (sensory ganglion, Herbst, Walter), exerts a stim ulus necessary for the complete regeneration of normal structures, on the contrary other workers have attributed less and less importance to these influences. The intermediate position that the influence of the nervous system is indirect, being exerted mainly through the con trolling of motor activity, is well expressed by Child (1905a) in the statement concerning anterior regeneration in Leptoplana, that, "as in posterior regeneration, there is close parallelism between the rapidity, amount, and completeness of anterior and lateral regeneration and the characteristic motor activity of the part concerned." Goldfarb (1909) concludes from his experiments on newts, earth worms, and planarians that "these experiments * * * should make one cautious about accepting the view of the direct or even indirect influence of a nervous influence on regeneration." In all these studies the point at issue has been whether or not complete regeneration of typical structures is possible in the absence of any influences exerted through the central nervous system. An affirmative answer to this question is apparently held, at least by certain of these investigators, to settle finally the question of nervous influence without any consideration being given to the comparison of the course of the regenerative process in animals in which the nervous system was removed from the regenerating area and those in which the nervous system had been uninjured in the portion of the animal left to regenerate. In only relatively few animals can the nerve-centers be removed without bringing about the destruction of, or degenerative changes in, other intimately connected portions of the nervous system, so that this type of operation has not been frequently undertaken.
Zeleny (1907) and Stockard (1908) removed the marginal sense organs from the disk of Cassiopea xamachana to determine the influence of these structures on the rate of regeneration. Both report that there was no evidence of any regulatory influence. In Zeleny's experi ments the entire margin of the disk with its sense-organs was removed and the rate of regeneration in these individuals compared with others in which the bell-margin and sense-organs were intact. In Stockard's experiments the results obtained from specimens prepared as above described were supplemented by those obtained with individuals from which one half of the marginal sense-organs was removed, while from the other half an equal amount of tissue was cut from between the sense-organs. The two halves were insulated by the removal of two diametrically opposite strips of subumbrella ectoderm. In both experiments the rate of regeneration was measured inward from the periphery of a cavity in the center of the disk from which a circular piece of tissue had been removed.
In both these researches the experiments were carried out with the view of ascertaining the influence of muscular activity and thus indirectly of the nervous system on the rate of regeneration. In each case it was held that there was no constant difference in the rate of regeneration between the active and inactive individuals.
In the course of my studies, which were taken up primarily to reexamine the work of Stockard and Zeleny upon this point, I dis covered that the marginal sense-organs influence regeneration inde pendently of their control of muscular activity. Such an influence of the sense-organs can be accounted for either on the ground that metabolic activities, not expressed by muscular activity, are under the control of the sense-organs, or that a direct trophic influence is exerted by the sense-organs on the regenerating tissues. A series of deter minations with the " biometer" of the rate of production by specimens under different experimental conditions, for which I am indebted to Dr. S. Tashiro, shows that the first of the two alternatives just mentioned offers a satisfactory explanation of the observed facts.
The course of normal regeneration as shown when a disk of tissue is removed from the center of a medusa disk is that first the ectodermal epithelium from both the exumbrella and subumbrella surfaces grows inwardly over the exposed surface of the mesoglcea until that extending from the two surfaces has come in contact. Fusion of the two layers takes place at once and then a sheet of new tissue begins to extend across the cavity left by the removal of the disk of tissue. In this sheet of new tissue no mesoglcea layer is at first distinguishable, the two layers of epithelium being in direct contact at their inner surfaces.
Just as in the embryonic development of all Cnidaria, the mesogloea makes its first appearance as an acellular layer of gelatinous material secreted by the cells of the layers it is to separate. In later stages of regeneration the newly formed mesoglcea fuses insensibly with that of the body of the disk at the periphery of the cavity. When the sheet of regenerated tissue reaches the center of the cavity its edges fuse, but leave for more than 24 hours a recognizable scar at the point where fusion took place. In the specimens used in the following experiments this scar was at first eccentrically placed, but within the first day after it had been formed it came to lie centrally in the sheet of new tissue. From this time the most noticeable change was the increase in the thickness of the regenerated tissue, which within two weeks—in speci wens in sea-water—could not be distinguished from the surrounding tissue. Long before this time tissue differentiation had taken place, so that functional nerves, muscles, gland and sensory cells had been formed from the syncitial mass into which the epithelial layers had been converted in the earliest stages of regeneration.