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. Favur- 
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 Daphnidz and Rotifera at certain seasons of the year, the partheno- 
genetic egg forms only one polar cell, and the animal developing from 
such an egg is tnvariably female, or more correctly 2? (¢), the male 
character being recessive. In other words, the daughter produced by 
parthenogenesis is exactly ike 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 
ofspring. 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 straightway after the formation of a single 
polar cell, usually while still within the body of the mother, and without 
awaiting the occurrence of a second maturation division. No segrega- 
tion of sex-characters has yet occurred within the egg, which 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. The 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 the mother, and a male is produced. But if the egg is fertilized, 
it takes up yolk and acquires a resistant shell, which ordinarily prevents 
