288 C. M. CHILD. 



shown in the paper on the asexual cycle of this species referred 

 to above that senescence is manifestly associated with growth in 

 each generation and that the occurrence of asexual reproduction 

 in this species is a result of the decrease in rate of metabolism 

 which occurs in the course of senescence. In that paper it was 

 also shown that asexual reproduction, which is essentially a process 

 of isolation of pieces and their reconstitution into whole small 

 individuals, brings about rejuvenescence as a result of the re- 

 organization and reduction which occur in the process of recon- 

 stitution. Evidently the animals undergo a regression to a 

 comparatively early stage of development with each reconstitu- 

 tion and it is also evident that the degree of rejuvenescence in each 

 reproduction is on the average the same, for the stock does not 

 as yet show any indication of a progressive senescence from 

 generation to generation. 



Whether gradual, progressive senescence of the stock is entirely 

 eliminated by the process of reproduction or will sooner or later 

 become apparent with continued asexual breeding can of course 

 be determined only by further breeding of the stock and it is 

 the writer's intention to continue the experiment as long as seems 

 necessary. But the apparent absence of sexual reproduction 

 in this species under natural conditions, to which attention was 

 called in the earlier paper, constitutes strong evidence for the 

 conclusion that the species is able to maintain itself indefinitely 

 by asexual reproduction alone. Since it is demonstrated that 

 asexual reproduction brings about rejuvenescence in this species 

 as well as in Planaria dorotocephala there is no apparent reason 

 why asexual reproduction should not continue indefinitely 

 without sensecence of the stock or race. All that is necessary 

 for the realization of this possibility is that the degree of rejuven- 

 escence in each generation should be on the average the same and 

 that is apparently the case. 



II. REJUVENESCENCE BY STARVATION. 



Planaria velata like P. dorotocephala and other species of 

 Planaria undergoes reduction in size when starved and this 

 reduction may be continued until the animal is but a small 

 fraction of its original size. The reduction in size is of course 



