EFFECT OF TIME INTERVAL BETWEEN SEPARATION OF DISKS INTO . HALVES AND REMOVAL OF SENSE-ORGANS.
In all the experiments on regeneration involving half-disks which retained their sense-organs it was noticeable that the difference in rate of regeneration from the two halves was most marked during the first day (see figures 6, 7, 8, 9). After that time the rates of the two halves gradually became more nearly equal, until at the end of an experiment, if the amount to be regenerated was large, there was only a alight difference between the rates of the two halves.
In a series of experiments designed to test the importance of the time factor in the difference in the rate of regeneration, sets of 10 disks each were separated into halves and the sense-organs were removed from one of the halves of each pair at intervals of 2 hours. It was soon found that no effect was noticeable if the centers were removed in less than 18 hours, and in most experiments 25 or more hours were necessary in order that the rate of regeneration should be the same from each half of a disk. When the second operation was made in less than 24 hours from the time of dividing the disk it was impossible to distinguish between such specimens and the controls from which half of the sense organs were removed at the time of the first operation.
Table 5 shows the effect of the removal of the sense-organs from a half-disk of a pair at different intervals after the separation of the disks into halves, each series consisting of 10 specimens.
As shown above, the line of demarcation between the time whe0i; influence of the sense-organs will be appreciable and that after which their removal has no effect is sharply drawn at an interval of about 25 hours. In each series of disks there was observed the same irregularity in physiological activity that was shown among the members of any large series. The individuals of the highest physiological activity, as shown in the rate of regeneration, were the first to show the effects of the sense-organs. In any series those specimens from which the sense-organs could first be removed with the assurance that their influence would be shown on the later stages of regeneration could be distinguished after 12 hours by observing the rate at which regeneration was taking place.
The foregoing data seem to show conclusively that there is a clearly marked influence of the sense-organs upon the rate of regeneration in Cassiopea. There is no evidence, however, that the presence of this influence is necessary for the formation of normal structures in regenera tion, as Herbst (1896 and 1899) concluded from his experiments on decapods; or as maintained by several of the earlier workers upon regeneration in platodes, and by Walter (1911) for Triton. The second of these special cases has been shown by the studies of Morgan, Child, and Goldfarb to be conditioned by influences other than the presence of the nerve-centers. The work of Steele has shown also that in several species of crustace,a the removal of an eye-stalk is not followed by the regeneration of a heteromorphic structure.
While none of these workers has laid any stress on the fact that the nervous system exerts an influence on the rapidity of the early stages of regeneration, it has been noted in several instances that the initial stages of regeneration are more rapid in the control animals than in those from which the nervous system has been removed. Thus, Goldfarb (op. cit., page 664) states that in salamanders the hind limb develops more slowly on the side from which the dorsal ganglia inner vating the leg had been removed than does that of the opposite side in which the ganglia remained when the spinal cord had already been removed. In a table on pages 665 and 666 he shows that this result was observed in all the specimens recorded save two, in one of which the regeneration was, at the time of measurement, equal for both legs, while in a single specimen the regeneration was most rapid from the side from which the ganglia were removed.