618 ROBERT H. BOWEN 



is free from cytoplasm (tigs. 39 to 41), exactly as in other 

 insect sperms. 



After moving into its definitive position the acrosome con- 

 tinues to elongate, and eventually becomes spun out into 

 a remarkably long, delicate apical piece (figs. 33 to 41). Fig. 41 

 is the latest stage in which I have seen the acrosome in anything 

 like its entirety, but the head of the sperm itself is still in an 

 intermediate stage of elongation. The extraordinary size 

 relations of the acrosome in the mature sperms can be inferred 

 from Meves' (1903) fig. 152. (See also his descriptive account.) 



To go back now to the early elongation stages, the structural 

 features of the acrosome itself deserve further attention. 

 Once the acrosome has become markedly elongate I have found 

 it as a rule impossible to differentiate the vesicular material 

 which is obscured by the heavily-stained ' granule '. However, 

 in cross-sections, especially of intermediate stages in elonga- 

 tion, the two materials can be readily distinguished (fig. 34), 

 and there can be no doubt that the two original constituents 

 of the acrosome remain distinct at least for a long period. 

 A comparison of figs. 32 and 33, and 36 and 37, mth fig. 34, 

 suffices to explain the appearances presented by the acrosome 

 when viewed from different aspects. It will be evident that 

 in figs. 33, 37, and 38 the acrosome is turned so that the 

 acrosomal granule is seen in plane view, while in figs. 32, 36, 

 and 39 the acrosome is seen in what one might call a side view 

 (compare fig. 34). As the figures show, the side of the acrosome 

 on which the vesicular material is disposed is usually turned 

 towards the major axis of the sperm. 



Within the acrosome itself various changes take place, the 

 first of which to be noted is the production, at the temporarily 

 attached end, of a clear area, which seems to be the first part 

 of the acrosome to free itself from the nuclear membrane 

 prior to migration (figs. 26 to 30). This clear zone is subse- 

 quently lost, and another one develops at the permanently 

 attached end of the acrosome, while at the very point of 

 attachment a darkly-stained body appears (figs. 31 to 39). 

 Whether this represents a differentiation of the acrosome. 



