ASEXUAL BREEDING AND PREVENTION OF SENESCENCE. 287 



emerged during September and October and after feeding grew 

 rapidly and in about a month attained full size and began to 

 fragment and encyst again. The worms of the third generation 

 emerged from the cysts in three to four weeks and again attained 

 full size in a little over a month and fragmentation and encyst- 

 ment began for the third time. 



Up to the present time this cycle has continued to repeat itself 

 more or less rapidly according to the frequency of feeding. 

 During the last year the animals have been fed only three times a 

 week instead of every day and growth has consequently been 

 less rapid. For two months in the summer of 1913 the stock 

 was placed in the refrigerator at a temperature of 8-10 C. in 

 order to avoid possible loss from high temperature, and during 

 one month of this time the animals were not fed. This pro- 

 cedure had, however, no other effect than to bring about frag- 

 mentation in a few individuals and a slight degree of reduction 

 from starvation. The few fragments were discarded and in 

 September the animals were brought back to room temperature, 

 feeding was resumed and growth began again. 



In this manner the stock has been carried through thirteen 

 asexual generations in less than three years. 'At present (March, 

 1914) the thirteenth generation is approaching fragmentation 

 and encystment. 



The stock shows no indication of loss of vigor. The animals 

 which emerge from the cysts in each generation show a high 

 rate of metabolism and are physiologically young and undergo 

 senescence in each generation in the same way as the animals in 

 nature. During the period of breeding there has been no 

 indication of the development of sexual organs so far as could be 

 determined by external examination. Certainly no genital 

 openings have ever developed and no eggs have been laid. 



As compared with the almost four thousand asexual generations 

 of Paramecium bred by Woodruff the number of generations 

 attained by this stock is small but the fact that a metazoan 

 species is capable of passing through twelve asexual generations 

 without any indications of sexual reproduction is of interest. 



Moreover, it is not because the animals do not become old that 

 they are capable of continued asexual reproduction. It was 



