INHERITANCE IN ASEXUAL REPRODUCTION 



185 



never been seen in this locality. The fungus mentioned by Rand 

 ('99) is evidently not a parasite of Hydra. 



Finally, there is no direct evidence bearing upon the question 

 of the effects of the age of the clone upon its variations. It has 

 been impossible to secure clones of known age since in this locality 

 sexual reproduction in H. viridis seems to be almost completely 

 suppressed. Hydras from wild populations have been examined 

 at all seasons during the past three years and in this time not one 

 with ovaries and only seven with spermaries have been found. 

 The treatment by which Whitney ('07) induced sexual repro- 

 duction was tried but without success. These facts seem to 

 justify the conclusion that sexual reproduction plays but little 

 part in the life history of the green polyp. 



BSf- 



Fig. 6 The number of polyps (ordinates) which died at the ages in days 



recorded on the abscissae, during Hase's experiments. ( ) H. fusca, ( ) 



H. grisea. 



In the development of theories of 'life cycles' in micro-organisms the 

 phenomena of 'depression' in Hydra have had a prominent place. 

 R. Hertwig, in particular, has sought to emphasize the relations of 

 'depression' to sexual reproduction, but the work of Frischolz ('09) 

 seems to prove that no such relation exists. Practically all studies of 

 depression have been carried out with polyps in mass cultures and 

 there is no evidence that the epidemics of depression reported were not 

 due to environmental rather than to internal factors. The ease with 

 which depression may be induced by unfavorable environment and 

 cured by putting the polyps in clean water makes it practically certain 

 that depression is a pathological condition. 



The only other work bearing upon the question of age is that of Hase, 

 who studied the age of individual polyps. He found a mean age of 

 55.2 days for H. fusca and 94.8 days for H. grisea, with the distribution 



