LIFE CYCLE OF HYDATINA SENTA 159 



Whatever the process may be by which the hfe cycle is deter- 

 mined, it is clear that the cycle is affected by both external and 

 internal agents. Neither one alone is responsible, we have to 

 reckon with both. This is precisely the state of affairs which 

 Woltereck ('09) finds in the daphnians, and which is confirmed by 

 McClendon ('10) and in part by Papanicolau ('10). Both exter- 

 nal and internal agents, according to these writers, influence the 

 proportion of sexual forms. It will be profitable, therefore, to 

 review briefly other groups in which there occur phenomena simi- 

 lar to those in Hydatina, and try to discover whether the recent 

 findings in Hydatina will throw any possible light on these other 

 groups. We may ignore such cases as occasional conjugation 

 followed by long periods of simple fission without preceding con- 

 conjugation, as in infusoria; or the alternating sexual and asexual 

 modes of reproduction as exemplified by hydroids. These phenom- 

 ena may or may not be related to that of alternating sexual and 

 parthenogenetic reproduction in rotifers. This brief review may 

 therefore be limited to the two groups of daphnians and aphids. 



Regarding the daphnids, the question of alternating modes of 

 reproduction, often spoken of as one of sex-determination just 

 as in Hydatina, has been discussed by many workers, especially 

 in Germany, and the views held have been more extreme than in 

 the case of the rotifers. The first experimental work of any impor- 

 tance was that of Kurz ('74) who found that when the water in 

 which a colony of daphnids lived was slowly evaporated, sexual 

 forms appeared; but his experiment was not controlled. Schman- 

 kewitsch ('75) suggested that increase in the salt content of the 

 water could cause sexual individuals to appear, just as he found it 

 to produce morphological changes. Weismann ('79), after an 

 extensive study of the problem, largely carried out in the field, 

 rejected the findings of the two previous workers, and from his 

 own observations drew the sweeping conclusion that no external 

 agent in any way influences the occurrence of sexual individuals 

 but that these are associated with certain generations, or better, 

 with certain broods. It is further illustrative of Weismann's 

 firm stand for internal determination, that he rejected the direct 

 action of the external conditions as a means by which the associ- 



THE JOURNAL OF EXPERIMEMTAL ZOOLOGY, VOL. 10, NO. 2 



