1909.] 



Garnet ogenesis of the Gall-Fly, etc. 



101 



go, the results indicate that one kind of parthenogenetic female lays eggs 

 which undergo no maturation, or at least no reduction, while the other kind 

 lays eggs which undergo a maturation division, and in which the 

 segmentation mitoses show the reduced number (10) of chromosomes. It 

 has already been shown that in the male of the summer generation the 

 •spermatogonial divisions have 10 chromosomes, although the body-cells 

 have 20, so that it is probable that the eggs which have a maturation division 

 give rise to males, and it is possible that the 10 chromosomes seen in their 

 segmentation mitoses are bivalent. In the body-cells these would then split 

 into their univalent components, giving the 20 chromosomes observed, but in 

 the developing germ-cells the haploid number of bivalents would be retained 

 until the single spermatocyte division. In the summer females, on the other 

 hand, the oogonia show 20 chromosomes, and these are probably produced by 

 females which lay eggs that undergo no reduction. 



The complete proof of these facts must be left until a fresh supply of 

 material can be obtained in the spring. 



Summary and Discussion. 

 The chief results described in the foregoing pages are the following : — 



1. The Gall-fly, Neuroterus knticidaris, has two generations in the year, the 

 flies appearing in April and June respectively. The spring generation 

 consists exclusively of females, which differ considerably from the females of 

 the summer brood. Their parthenogenetic eggs are laid in oak buds, and all 

 the eggs laid by any one female develop into individuals of the same sex in 

 June, i.e. some of the spring females are male-producing, others are female- 

 producing. The summer generation thus consists of males and females ; 

 their eggs are fertilised, and are laid in the tissue of young oak leaves, and 

 give rise to galls very different from those produced in the spring. The flies 

 from these galls hatch in April and thus complete the cycle. 



2. Mitoses in the body-tissues of young pupae show about 20 chromosomes, 

 both in the spring parthenogenetic females, and the males and females of the 

 summer brood. 



3. In the spermatogonia of young male larvae, mitoses show 10 chromo- 

 somes. In the primary spermatocytes of young pupae 10 chromosomes 

 appear. An imperfect mitotic figure is developed, but the nuclear membrane 

 does not disappear ; and after the metaphase is reached the nucleus returns 

 to a " resting " condition. During this process the cell develops an elonga- 

 tion at one end, at the tip of which is one of the centrosomes (or centrioles) ; 

 as the nucleus re-forms, the centrosome and a small piece of cytoplasm are 

 separated off, as happens in the Bee and Wasp. The process is thus much 



