ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 569 



contraction, either in a vertical or a longitudinal direction, is sufficient 

 to give rise to the whole of the complicated movements, since the con- 

 traction arises from a single point in the wing-root Observation has 

 shown that the mechanism of flight varies greatly within the class 

 Insecta, so that the above conclusion cannot be applied to other forms 

 without investigation. 



Gametogenesis of Gall-fly.* — L. Doncaster gives the following 

 summary of his chief results. The gall-fly, Nmroterus lenticularis, has 

 two generations in the year, the flies appearing in April and June re- 

 spectively. The spring generation consists exclusively of females, which 

 differ considerably from the females of the summer brood. Their par- 

 thenogenetic 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 

 fertilized, 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. 



Mitoses in the body-tissues of young pupas show about twenty chro- 

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

 and females of the summer brood. In the spermatogonia of young 

 male larvae, mitoses show ten chromosomes. In the primary spermato- 

 cytes of young pupas ten 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 elongation 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 like that found in the bee, except that the nucleus returns to 

 a " resting " condition. 



The spermatocytes now develop about twenty chromatin masses which 

 form themselves into ten band-like chromosomes. These shorten, form 

 an equatorial plate across the cell, a typical spindle is produced ; and 

 the chromosomes divide so that ten travel into each daughter-nucleus. 

 Two spermatids are produced, which are similar, except that in some 

 cases at least one of them receives a small extra-nuclear body of unknown 

 nature which is absent from the other. Both spermatids develop into 

 spermatozoa. In primitive ova in the young female larvae of the summer 

 generation mitoses like those in the body-cells are found, with apparently 

 twenty chromosomes. After the deposition of yolk has begun, no further 

 nuclear divisions occur in the egg. 



The maturation divisions of the summer eggs are difficult to follow, 

 but apparently two divisions occur, giving rise to four groups of chromo- 

 somes, of which the three outer represent the three polar nuclei, while 

 the innermost group sinks in to form the female pronucleus. This 

 group probably consists of ten chromosomes, but they are so crowded 

 that the number commonly appears rather less in sections. 



The male pronucleus and the female pronucleus meet and form the 



* Proc. Roy. Soc, Series B, lxxxii. (1910) pp. 88-113 (3 pis.). 

 Oct. Wth, 1910 2 Q 



