The experiments with Mg solutions show that there is an influence of the sense-organs on the rate of regeneration which is apparently exercised for a considerable time after muscular activity has been suppressed. It was impossible, however, by this method to differentiate with any certainty between the two effects, since there is no visual means of ascertaining the exact time at which the sense-organs lose their power of sending out the stimuli necessary for normal contraction. Since, as was shown by Mayer (op. cit.), it is possible to maintain a circuit wave of contraction in the muscles of a half-disk without sense organs by making a series of cuts by which an endless labyrinth of subumbrella tissue is formed (figs. 4 and 5), the part played by muscular activity and that by the stimuli from the sense-organs can be directly compared. In such experiments all the sense-organs were removed from the medusa disks, the halves insulated, and a circuit wave of contraction started by an induction shock in a labyrinth cut in the muscle-tissues of one of its halves. Once established, the contraction wave would be maintained throughout the course of an experiment unless interrupted by an unusually strong stimulus through some accident in handling. When interrupted in this way the circuit wave could be established again by renewed electrical stimulation. The amplitude of the contraction wave becomes gradually reduced as time goes on, but there is little variation in its rate. When the rates of regeneration of the halves of any disk prepared in this manner are compared, it is found that the half in which the circuit wave is main tained regenerates slightly faster than the inactive one. This differ ence in rate is, however, very much less than between the halves of a disk from one-half of which the sense-organs have been removed (compare figs. 6 and 8), although the activated disk pulsates from 3 to 10 times as fast as the one under the control of the sense-organs.
The amount of activity and metabolism in the muscles if they have any noticeable influence on the rate of regeneration ought to produce a clearly demonstrable result, but as shown by the data in table 3 and figure 8 the difference is relatively small. From the point of view of the chemical nature of metabolism (including regeneration) the difference in temperature might conceivably be sufficient to account for the observed difference in rate of regeneration. The half-disks in which the circuit wave is maintained show a greater regularity in the rate of regeneration than do the active disks recorded in table 1.
Further demonstration of the influence of the sense-organs on the rate of regeneration is furnished in another series of experiments in which the two insulated halves of a disk are compared, one of which is contracting normally under the influence of its sense-organs, while all the sense-organs are removed from the other half and a circuit wave of contraction maintained in a labyrinth of its subumbrella muscles. In
a disk prepared in this manner the rate of pulsation of the activated half will vary from 3 to 10 times that of the normally contracting half.
The comparative rates of regeneration for 40 disks under these conditions are shown in table 4 and figure 9.
As was pointed out previously, the stimulus for pulsation originates, at any given tune, from a single sense-organ, and pulsation will be continued at nearly its normal rate by a medusa upon which a single sense-organ remains. To test the efficacy of a single sense-organ to control activities other than muscular contraction a series of disks prepared as above described (except that each "active" half-disk had a single sense-organ) were used in a parallel series of regeneration experiments. In all instances such active disks showed an unusual irregularity in the rate of pulsation, since there was present no other nerve-center to take up the initiation of impulses causing pulsation whenever the rate of discharge of the single organ fell below normal.
There was noticeable an indefinite sort of rhythm in the rate of pulsation, as if the sense-organ underwent periods of fatigue from which it was enabled to recover after having given off the stimuli for pulsation for a time at a rate considerably lower than that observed previous to the removal of the other centers. Those periods of depression in the pulsation-rate were of such short duration that they could not be distinguished in the amount of tissue regenerated, if, as would seem probable, the sense organs were affecting other metabolic activities in a similar manner.
In all experiments of this type the results were of the same nature as in those when the sense-organs on the activated half-disks had not been disturbed. The amount of time necessary to bring about the closing of the cavity in which re generation was measured was not sufficiently different from that in specimens where the active half-disk retained its sense-organs to indicate any serious distur bance in metabolic activity. The different specimens in these, as in the previous series, showed great varia tions in physiological activity (see tables 1, 2, 3, and 4), but there was noth ing strikingly characteristic about the behavior of the specimens with a single sense-organ.