194 Chromosomes in Pier is and Abraxas 



The second spermatocyte division is easily recognisable by the 

 smaller size of the cells and of the chromosomes (Figs. 8, 9). Fifteen 

 chromosomes can always be seen in well-placed equatorial plates, of 

 which usually four, sometimes only three, and occasionally as many 

 as five, are noticeably smaller than the rest. The two smallest are 

 often, but not always, lying side by side. In some figures, these two 

 small ones are very conspicuous ; in others, often in the same follicle, 

 only one very small one is found. This led me at first to suppose 

 that in the first spermatocyte division there must be an unequal pair 

 of heterochromosomes such as Wilson has described in Lygaeus and 

 Euschistiis, but if it exists, the difference in size between the two 

 members of the pair is not sufficient to reveal itself in a side view 

 of the metaphase and early anaphase. I have examined many such 

 figures with great care, and have never found a dividing chromosome 

 in which the two halves were certainly unequal. In some figures a 

 very slight inequality is suggested occasionally, but in others, where 

 every chromosome is visible, no inequality can be seen. Further, the 

 fact that there may be three or four all equally small, and in other 

 equatorial plates in the same follicle only one (or rarely none at all) 

 which is conspicuous by its small size, makes the interpretation of 

 these difierences as being due to unequal heterochromosomes very 

 doubtful. It may be concluded therefore that in both sexes of Pieris 

 jbrassicae the somatic number of chromosomes is 80 ; these vary in size 

 and include one pair which differ from the rest in the growth-phases 

 in constituting a "chromatin-nucleolus," so resembling heterochromo- 

 somes. The reduced number in both sexes is 15, and although there 

 is a suggestion that the heterochromosomes form an unequal pair in 

 the male, the evidence for this is quite inconclusive, and the appearance 

 is very probably deceptive. 



Abraxas grossulariata. 



The material consisted of ovaries removed from larvae varying from 

 somewhat more than half-grown up to nearly full-grown, and treated 

 similarly to those of Pieris. The ovarian tubes are shorter, but the 

 stages follow each other nearly as regularly as in Pieris. The oogonia 

 contain a nucleolus more or less conspicuously double, which consists 

 chiefly if not entirely of chromatin. Among the oogonia, especially 

 near the apex of the tube, groups of cells are constantly found under- 

 going degeneration ; their number is often considerable. 



Oogonial mitoses, even in rather young larvae, are less numerous 



