ASEXUAL BREEDING AND PREVENTION OF SENESCENCE. 29! 



with this food the animals in general attain a larger size before 

 ceasing to feed and undergoing fragmentation than when fed 

 with liver. But when earthworm is given in sufficient quan- 

 tities senescence occurs, though its course is somewhat different 

 from that of senescence with liver as food. The effects of differ- 

 ent foods on the course of the life cycle will be discussed at another 

 time. 



During the months of July, August and September of 1913 the 

 stock was kept in a refrigerator at a temperature of about 10 C. 

 in order to avoid the danger of encystment from high tempera- 

 ture, and feeding during this period was of course less frequent 

 since the animals were less active and required less food to 

 maintain a constant size. During September they were not 

 fed at all and underwent reduction to a somewhat greater extent 

 than usual in the starvation periods. At the beginning of Oc- 

 tober they were brought back to room temperature and since that 

 time the feeding has been continued as before. 



The five worms which now make up the stock are in the same 

 generation as they were two years ago and of somewhat smaller 

 size, about five millimeters, than at the beginning of the experi- 

 ment. They are active, behave like young animals, react very 

 strongly to food and appear in every respect to be as young physi- 

 logically as growing w r orms of the same size. Unfortunately the 

 small size of the stock has not permitted determinations of sus- 

 ceptibility at intervals, but judging from the activity and ap- 

 pearance of the animals their susceptibility would be that of 

 young animals. While these animals have remained in the same 

 generation during more than two years and are to all appearances 

 as young physiologically as at the beginning, in fact somewhat 

 younger, since they are kept at a smaller size than when first 

 isolated as a stock, the other portion of the same generation 

 which \vas used for asexual breeding has in the same length of 

 time passed through twelve asexual generations with the cycle 

 of high susceptibility, growth with decreasing susceptibility, 

 cessation of feeding, fragmentation, encystment, reconstitution 

 in the cysts and emergence in each generation. In these partially 

 starved animals the changes characteristic of the life cycle have 

 been inhibited and they have remained at practically the same 



