In tadpoles from which the caudal portion of the spinal cord had been removed regeneration of the tail took place more slowly than in the control animals in which the cord was uninjured (p. 672). Again, concerning earthworms from which the head had been cut off and several millimeters of the ventral cord removed he says (p. 708): "The head regenerates rather later in these operated animals than in control animals." In the regeneration of the arms of the starfish (p. 711) a similar observation is recorded.
Goldfarb, however, is of the opinion that "any severe injury * * *, whether involving the nerves or any other tissue, retards regeneration." Stockard, on the other hand, concluded that in Cassiopea, as Morgan had already shown for a considerable number of animals, the rate of regeneration increased in proportion to the extent of injury and that the deeper the cut—i.e., the nearer to the center of the disk—the faster would be the following regeneration.
In my experiments the amount of tissue removed from the margin of each half of any disk was the same. The differences in result observed were, therefore, due to the difference in kind, not in quantity of tissue removed. As recorded previously, the difference in rate of regeneration is in Cassiopea greatest in the early stages and gradually declines throughout its course, at least through the periods followed in these experiments. Goldfarb's observations appear also to show a similar course of events in earthworms, starfish, and amphibians. This result, on the other hand, is opposed to the conclusion of Child (1910) that "as most experiments not only on the Turbellaria, but on other forms, indicate, it is probable that the early stages of the forma tion of new tissue are largely or wholly independent of the nervous system." A later part of the same statement is in perfect accord with my result, as follows: "But it is difficult to understand how the nervous system of an adult animal could fail to affect the amount and rapidity of growth in a regenerating part composed largely of muscles and sense-organs. Absence of such an effect would be in direct
opposition to the well-established fact of the functional influence of the nervous system on various parts of the organism." Morgulis (1912) concludes from his experiments on brittle-stars that the presence at the cut surface of the radial nerve, either with or without its being in continuity with the remainder of the nervous system, is a " conditio sine qua non" for normal regeneration and that the pres ence—purely as a mechanical matter, not the functional activity— of the nerve is the important factor in regeneration. This is in direct opposition to my results. A study of the figures illustrating his paper shows, nevertheless, that the arms in which the nervous connection is undisturbed (control) regenerates most rapidly.
As to the nature of this influence, it is evident from the study of the rate of general metabolism of half-disks with and without sense-organs that it is closely associated with if not identical with the control of the general metabolism of the animal. The latter experiments have not been carried far enough to give a definite answer to the question of whether or not there is a gradual decline in the difference in the rate of metabolism corresponding to that shown in regeneration.
This result clearly supports the general contention of Child, that the influence of the nervous system on regeneration is indirect rather than direct, but does not confirm his statement that there is a direct relation ship between the rapidity of regeneration and the "characteristic motor activity of the parts concerned." It is shown, on the contrary, that motor activity may be greatly increased without altering to a proportional extent either the rate of regeneration or general metabo lism.
The rate of regeneration appears to be simply one expression of the general metabolic activity of an organism and consequently to be subject to the control of the nerve-centers in the same manner as the many other functional activities—for some of which, at least, a direct nervous control can not be denied.