144 David Day Whitney 



very much smaller than in the other two kinds of eggs and usually 

 were so crowded together that it was impossible to count them 

 except in a very few cases. 



These observations show that there is probably a reduction in 

 the number of chromosomes in the male parthenogenetic and 

 winter egg but no reduction in the female parthenogenetic egg. 

 The former case would be similar to what occurs in the honey- 

 bee. In the aphids Stevens found that there is no reduction in 

 the number of chromosomes in either of the male or female par- 

 thenogenetic eggs but only in the fertilized egg. 



It appears that in different animals parthenogenetic eggs vary 

 in the number of polar bodies that they give off. The male egg 

 of Asplanchna, Hydatina and Apis gives off two polar bodies while 

 the male egg of aphids gives off only one. 



If it is true that the male egg when fertilized becomes the winter 

 eggwhich develops into a female it seems evident that the reduction 

 in the number of chromosomes and the formation of the second 

 polar body is not in itself the factor that determines the ultimate 

 sex of the egg. 



The sperm would seem to introduce a factor that determines 

 the sex of the embryo. This idea is strongly suggested by the 

 evidence that Meves has brought forward in the case of the honey- 

 bee in which he finds that only one kind of functional sperm is 

 produced. Morgan also finds a similar phenomenon for certain 

 Phylloxerans. If the same process occurs in the sperm of Hydatina 

 the cause of the change in sex of the male egg may be at least 

 surmised. 



VII SUMMARY 



1 In the female parthenogenetic egg of Hydatina senta there 

 is no reduction in the number of chromosomes during maturation. 

 One polar body is extruded. 



2 In the male parthenogenetic egg there is a reduction in the 

 number of chromosomes during maturation. Two polar bodies 

 are formed, one of which subsequently divides. 



3 In the winter egg, that becomes fertilized, there is a reduc- 

 tion in the number of chromosomes during maturation, and since 



