CASTLE: THE HEREDITY OF SEX. 199 



number of cases, sexual reproduction (by fertilized eggs) occurs in the 

 same species with parthenogenesis, the two processes either alternating 

 with each other, or occurring under different external conditions. Favor- 

 able conditions in such cases result in parthenogenesis ; unfavorable con- 

 ditions of any sort may result in sexual reproduction. 



1. With a single exception to be discussed presently, we know that in 

 uninterrupted parthenogenetic reproduction, as it occurs, for example, in 

 the Daphnidse and Rotifera at certain seasons of the year, the partlieno- 

 genetic egg forms only one polar cell, and the animal developing from 

 such an egg is invariably female, or more correctly 9 ((?), the male 

 character being recessive. In other words, the daughter produced by 

 parthenogenesis is exactly like her mother. No segregation of sex-char- 

 acters has taken place in her development. That the male character is 

 still present in the agamic female is known from the fact that such a 

 female retains the capacity to produce males under appropriate external 

 conditions. 



2. At the return to sexual reproduction, the parthenogenetic mother 

 produces eggs which form a second polar cell, and from such eggs (if 

 unfertilized) only males develop. It is clear, then, that in the second 

 maturation division the female character has been eliminated from the 

 egg, for were it still present there, it must from its nature dominate. 



In the honey-bee, all the eggs without exception form two polar 

 bodies, and the unfertilized egg invariably develops into a male. Ac- 

 cordingly a queen-bee which has not copulated can produce only male 

 offspring. But one which has copulated produces both male and female 

 offspring, the former, however, only from unfertilized eggs, the latter 

 always from fertilized eggs. 



In parthenogenetic Rotifera and Crustacea, under optimum external 

 conditions, the egg develops straightv\^ay after the formation of a single 

 polar cell, usually while still within the body of the niDtlier, and without 

 awaiting the occurrence of a second maturation division. No segrega- 

 tion of sex-characters has yet occurred within the egg, whicli develops, 

 without the necessity of fertilization, into an agamic female like the 

 mother. If, however, external conditions are unfavorable, the egg will 

 not proceed to develop until it has undergone a second maturation divi- 

 sion. Tlie egg is then capable of development either with or without 

 fertilization. If it is not fertilized, as must necessarily be the case unless 

 the mother has copulated, development takes place at once within the 

 body of tlic mother, and a male is produced. But if the egg is fertilized, 

 it takes up yolk and acquires a resistant shell, which ordinarily prevents 



