182 HEREDITY AND SEX 



eggs and others small eggs ? There must be either two 

 kinds of stem mothers or one kind with double po- 

 tentiahty. Inasmuch as in other parthenogenetic types 

 there is experimental evidence to prove that environ- 

 mental conditions determine which alternative state, 

 whether male-producing or female-producing individ- 

 ual, is realized, so here we may, provisionally, follow 

 the same interpretation. Once the course is deter- 

 mined the subsequent internal events follow for two 

 generations in a definite order. If the stem mother has 

 been affected in one way, all of her daughters produce 

 large eggs ; if in the other way, small eggs. 



In another group of animals, the daphnians, parthen- 

 ogenetic species occur, that, in certains respects, are 

 like the phylloxerans ; but these species illustrate also 

 another relation of general interest. 



The fertilized winter egg produces always a female, 

 the stem mother, which gives rise by parthenogenesis 

 to offspring like herself, and the process may continue 

 a long time. Each female produces one brood, then 

 another and another. The last broods fail to develop, 

 and this is a sign that the female has nearly reached 

 the end of her life. 



But a parthenogenetic female may produce one or two 

 large resting eggs instead of parthenogenetic females, 

 and the same female may at another time produce a 

 brood of males. The large resting eggs are inclosed 

 in a thick outer protecting case. They must be fer- 

 tiUzed in order to develop, yet they do not develop at 

 once, but pass through an enforced, or a resting, stage 

 that may be shortened, if the egg is dried and then 

 returned to water. 



