1909.] 



Gametogenesis of the Gall-Fly, etc. 



103 



especially the Bee and other Aculeata. The life-history in the gall-flies is 

 more complicated, involving an alternation of two different generations, but 

 in some respects this simplifies our understanding of the facts. In the Bee 

 and its relatives, fertilisation of the egg is facultative, and to all appearance, 

 at least, the maturation of the egg is similar, whether fertilised or not. 

 In the gall-flies the parthenogenetic eggs belong to a different generation, 

 and since they give rise to both sexes, sex-determination in this generation, 

 at least, does not depend on fertilisation. In the summer brood all eggs 

 are fertilised and all give rise to females, so that here the gall-flies resemble 

 the Bee. As in the Bee also, in the spermatogenesis there is only one 

 maturation division of the nucleus, the number of chromosomes in the 

 spermatogonial mitoses being the same as that entering the spermatids; in 

 each case the first spermatocyte division is abortive, only a small portion 

 of the cell being extruded. The Gall-fly differs from the Bee, but resembles 

 the Wasp and Ant in producing two complete spermatids from each 

 spermatocyte, while in the Bee one of the daughter nuclei degenerates 

 without becoming a spermatozoon. -It is tempting to speculate on the 

 reason for this peculiarity in the Hive-bee, and a suggestion with regard 

 to it will be made below after the parthenogenetic generation has been 

 considered. 



In the spring generation of Neiiroterus, the eggs of which develop 

 parthenogenetically, the evidence is unfortunately not yet complete. It has 

 been seen that the eggs of some individuals give rise to females, those of 

 others to males. The microscopical evidence, so far as it goes, suggests that 

 in the eggs laid by some females there is possibly no maturation division, 

 and in any case these eggs contain the diploid number of chromosomes ; in 

 eggs laid by other females a maturation division occurs, and the segmentation 

 mitoses show the halved (haploid) number. It is suggested that the former 

 type of egg develops into a female and the latter into a male, hut since there 

 is possibly only one maturation division in these, and since the body-cells 

 of male pupae contain the diploid number of chromosomes, it is possible 

 that the chromosomes of the segmentation mitoses are bivalent. In the 

 Bee also the body-cells of the male contain more chromosomes than the 

 spermatogonia, in this case apparently four times as many (about 64 instead 

 of 16).* 



If it is the fact that parthenogenetic eggs of Neuroterus which have the 

 diploid chromosome number in their segmentation mitoses develop into 

 females, and those which undergo reduction become males, the sex- 

 determination may be imagined as follows : — 



* Meves, 'Arch. Mikr. Anat.,' vol. 70, 1907, p. 420. 



