386 SENESCENCE AND REJUVENESCENCE 



indicates that physiologically they are very much older than the 

 latter. In consequence of continued feeding and growth and the 

 absence of the reconstitutional changes connected with fission, 

 these animals have evidently attained a stage of development which 

 is not reached by the animals in nature. 



The conditions in nature which prevent the animals from 

 attaining the later stages of development and sexual maturity are 

 less abundant food and consequently greater activity, which in 

 turn determines more frequent fission, so that the animals are 

 almost continuously undergoing reconstitution. Moreover, the 

 animals are subjected more or less periodically to periods of partial 

 starvation. Insufficiency of food may arise from the rapid increase 

 in numbers of the animals by fission during the summer, perhaps 

 also from the slow reproduction of the food animals in winter. 

 These two facts, fission and repeated partial starvation, contribute 

 to keep the animals physiologically young and so prevent them 

 from attaining the age and physiological condition in which sexual 

 maturity occurs. 



Planaria maculata becomes sexually mature in some localities 

 and not, or only very rarely, in others (Curtis, '02). In the latter 

 localities the factors which prevent the occurrence of sexual ma- 

 turity are undoubtedly the same as those which produce the result 

 in P. dorotocephala, i.e., repeated fission and periodical or occasional 

 partial starvation. 



In a stock of P. maculata kept in the laboratory I found that 

 sexually mature individuals, after egg laying, lose the sexual organs 

 and undergo a considerable reduction in size. During this period 

 they take little or no food, but after a time begin once more to feed 

 and grow, and if growth is rapid they may reproduce agamically, 

 while if it is slow they may in some cases become sexually mature 

 again without passing through any period of agamic reproduction. 



The differences in susceptibility of the animals at these different 

 periods indicate that the sexually mature stages are physiologically 

 older than others, and that after egg laying they undergo a consider- 

 able degree of rejuvenescence during the reduction, and once more 

 begin to undergo senescence when they begin to feed. Whether 

 they remain asexual or become sexually mature depends on the 



