Sex- Determination in the Gall-Fly, N. lenticularis. 



189 



Maturation of the Fertilised Eggs. 



The results of the experiments described make it certain that any individual 

 sexual female produces exclusively, or almost exclusively, male-producing 

 or female-producing agamic offspring. The experiments, therefore, disprove 

 the hypothesis that the two types of agamic female are due to dimorphism of 

 the spermatozoa produced by one male. Two possibilities then remain : the 

 two types of agamic female may be due to two kinds of eggs laid by different 

 sexual females ; or, if each sexual female mates with only one male, they may 

 be due to two kinds of males which produce different spermatozoa, as has 

 been suggested by Morgan in the somewhat analogous case of Phylloxera 

 mrycecaulis* I have carefully re-examined my preparations of the spermato- 

 genesis in a number of males, and can find no regular differences among 

 them ; while not definitely denying their existence, I am unable to find any 

 evidence for them. There is certainly no difference in chromosome number, 

 nor can I find that any chromosome is constantly larger or smaller in some 

 males than in others. It remained, therefore, to discover whether any 

 differences could be found among the eggs laid by different sexual females, 

 and on the analogy of the facts described in my earlier papers on the subject, 

 it seemed possible that such differences might be expected in the maturation 

 divisions. Some indication that such differences might exist was given in 

 the first paper, but as there was then no suspicion that the females were of 

 two types, the eggs of individual females were not preserved separately. It 

 was therefore necessary to collect fresh material. For this purpose, in the 

 spring of 1914, and again in 1915, a number of females were allowed to lay 

 separately, and their eggs preserved in Gilson's acetic alcohol sublimate 

 (absolute alcohol, glacial acetic acid and chloroform, equal volumes ; sublimate 

 to saturation) at various ages up to about four or five hours after laying. I 

 have cut sections of these eggs (I wish to record my indebtedness to 

 Mr. D. W. Cutler for valuable help in this part of the work) and have also 

 re-examined all the eggs preserved in 1906 and 1907, on which the description 

 in my previous paper was based. The account that follows is thus derived 

 from a series of over 300 eggs of 15 separate females, in addition to about 

 200 selected eggs of mixed females. I estimate, however, that fully half of 

 these are in stages either too young or too old to throw any considerable light 

 on the maturation process. 



The first suggestion of a difference in the maturation process of eggs of 

 different females was that in some eggs the polar chromosomes, after the 



* T. H. Morgan, " The Predetermination of Sex in Phylloxerans and Aphids," ' Journ. 

 Exp. Zool.,' vol. 19, p. 285 (1915). 



