INSECT SPERMATOGENESIS 611 



in the form of a thin layer on the nuclear wall (figs. 18 to 20). 

 The exact appearance depends, of course, on the orientation of 

 the nucleus with respect to the observer. This rearrangement 

 of the chromatin is again reminiscent of the hemipteran 

 spermatid (Bowen, 1922 a), with the difference that in the 

 Lepidoptera the clear area seems to be opposite the inser- 

 tion of the tail filament, rather than around it, as in Hemiptera. 

 This arrangement of the chromatin is very clear in Pygaera, 

 but made out with great difficulty, if at all, in my preparations 

 of Callosamia. Only in rare cases did the chromatin stain 

 with characteristic intensity in any of my preparations, the 

 fixation in Flemming without acetic acid being apparently 

 responsible for this. I have noted the same result in the testes 

 of other animals. It is an exceedingh^ fortunate failure, for 

 it allows of many observations which could not possibly be 

 made if the sperm head were intensely coloured. Occasionally, 

 especially in later stages, some of the heads in a cyst will stain 

 intensely (compare figs. 40, 41, and 60), a result which makes 

 easy the determination of the exact limits of the head itself. 



The division of the head into the staining and non-staining 

 areas noted above, seems to have been made out by Platner 

 (1889), but his figures do not give a very adequate idea of the 

 actual conditions. During the early stages in the elongation of 

 the acrosome, the clear area gradually disappears (figs. 21, 22, 

 and 26), and the head then stains uniformly (figs. 28, 29, &c.). 

 During these latter changes the head seems to undergo a diminu- 

 tion in size, a phenomenon which is met with not uncommonly 

 (always '?) in insect sperm formation. The nucleus, at first 

 spherical, gradually elongates (figs. 35 to 41 and 57 to 60), as 

 in other insect spermatids, and eventually becomes a long, 

 delicate rod, not unlike the sperm head in Hemiptera (Bowen, 

 1922 a). 



Aside from the differentiations already noted in the spermatid 

 nucleus, I have also constantly found within it a small darkly- 

 stained body, of spherical shape, which is perhaps of nucleolar 

 origin, possibly related to the intra-nuclear body which I have 

 described in the hemipteran spermatid (Bowen, 1922a). In 



