90 



Mr. L. Doncaster. 



[Sept. 21, 



Tlic Somatic Mitoses. 



In attempting to count the chromosomes in the mitoses in the egg, it was 

 found difficult to determine the exact number, and with a view to checking 

 this a number of counts were made of division-figures in the body tissues 

 {developing wings, hypodermis) of male and female pupoe. Some of these 

 mitotic figures are represented in Plate 1, figs. 1 and 2. In all cases the 

 chromosomes are nearly equal in size, and the number is certainly approxi- 

 mately 20 in somatic mitoses. In mitoses in male pupae or larvae just before 

 pupation, 50 counts gave an average of 19'2 chromosomes, 17 showing 20, 

 and the remainder 19 or 18. In female pupaj of the summer generation, 

 40 counts gave an average of IQ'-I, 25 showing 20, and the rest 19 or 18. 

 In the pupaj (female) of the spring generation, division-figures are scarce, as 

 development proceeds much less quickly, but it is clear that the number is 

 sensibly the same as in the summer brood (I'late 1, fig. 3). 



The fact that in 50 counts of male mitoses only 17 showed 20 chromo- 

 somes, while 25 out of 40 had that number visible in females,_i]aight suggest 

 the possibility that the male actually has one chromosome less than the 

 female. In cases such as this, where the chromosomes are small and crowded, 

 it is quite possible to count one elongated or bent chromosome as two, 

 but some of the mitoses with 20 are so clear as to leave no doubt that this 

 is the true numbei'. The difference probably depends on the fact that the 

 female pupa from which most of the counts were made had exceptionally 

 fine mitoses, and was very well preserved, so as to give a smaller proportion 

 of irregular figures than in the male pupae used. It is possible that in the 

 male one chromosome is smaller than the rest, and sometimes hidden, but 

 I have not been able to convince myself of this. 



Spermatogenesis. 



The larvte are found in tlie galls in tlie latter half of May ; they develop 

 very rapidly, for at the middle of the month they are exceedingly small, and 

 by the end most of the galls contain pupai. In very young pupa), with no 

 colour except in the eyes, tlio testes contain spermatocytes and the various 

 maturation phases ; a week later all the puptc are grey or black, and the 

 " meiotic pliase " is over, only spermatids and nearly ripe spermatozoa being 

 found. The full-grown larvtc also have spermatocytes in the testes, and some 

 follicles show prophases of tlie juaturation ; for the spermatogonia! stages, 

 half-grown larvae, al)out t'oui' or live days younger, are required. In very 

 joung larv.'c I have not been able to distinguish tlie gonads. 



Th(! larvic and ])up;e were opened with needles and ti.xed entire in 



