1 8 Hewitt, Cytologicnl Aspect of Parthenogenesis in Insects. 



have to be considered by Castle as exceptions to his 

 view (21, p. 200) that ' a segregation of sex characters 

 takes place at the formation of the second polar cell,' and 

 later, ' hence, if the egg which has formed two polar cells 

 develops without fertilisation, it must develop into a male.' 

 The two exceptions which he considers are RJiodites 

 rosae and Bonibyx mori. As has already been stated, 

 very few males are produced by the parthenogenetic eggs 

 of R. rosae. There is no reduction in the number of 

 chromosomes, so that there is no segregation of sex 

 characters. Castle admits this, and suggests that the 

 egg retains a capacity to eliminate the dominant female 

 character, which it does occasional!}', and so males are 

 produced, as in other parthenogenetic animals, under 

 appropriate conditions. (The italics are mine.) He finds 

 it necessary to bring in other conditions than the mere 

 segregation of male and female characters in the matura- 

 tion divisions. His treatment of the second exception, 

 that of B. vwri and L. dispar is still less convincing. In 

 the parthenogenetic eggs of these moths two polar bodies 

 are given off, as in the normal fertile &%^, and the small 

 proportion of these eggs which develop into perfect insects 

 are of both sexes. In attempting to explain this excep- 

 tion, he says (p. 205) ' But it is entirely possible that 

 in the very exceptional ^^^ which develops normally, 

 a second maturation division has for some reason failed 

 to take place, or after it has taken place, a reunion has 

 occurred of the second polar nucleus with the q^^ nucleus, 

 as sometimes in the &^^ of Arteniia, according to Brauer. 

 Such a reunion would bring together again the sex 

 characters segregated in maturation, and would produce 

 the physiological and morphological equivalent of the 

 cleavage nucleus of a fertilised egg. A similar result 

 would follow the complete suppression of the second 



