182 HEREDITY AND SEX 



eggs and others small eggs ? There must be, in all prob- 

 ability, two kinds of parthenogenetic eggs produced 

 by the stem mother — or at least there must be two 

 kinds after the single polar body has been extruded.^ 



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- 

 tilized 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. 



^ The explanation may be found in the occurrence of two types 

 of males — one type with two sex chromosomes, the other with one — 

 two such types were actually figured in my paper. From the type 

 with two sex chromosomes a stem mother would be produced with 

 four sex ehi'omosomes (two coming from the sexual egg). 

 She would give rise to migrants with large eggs. From the type 

 with one sex chromosome a stem mother would arise that produced 

 small eggs with three sex chromosomes. According to whether two 

 or one went out into the polar bodies of the small eggs, the two types 

 of male would be reproduced. 



