INSECT SPERMATOGENESIS 617 



practically untouched ; but as its later development offers 

 a number of interesting features I have thought it worth while 

 to work out the subsequent events in Callosamia and 

 Pygaera from a comparative standpoint. The acrosome in 

 Pygaera is particularly'- favourable for study because of its 

 large size. 



In Pygaera the acrosome. when finally deposited consists, 

 as noted above, of a clear vesicular portion, the acrosomal 

 vesicle, and a very large, darkly -stained granule, the acrosomal 

 granule. The latter is presumably contained within the former, 

 but the large size of the granule gives one the impression 

 rather of a bipartite mass (figs. 15, 16, and 17). The vesicular 

 material seems now to undergo further concentration, its out- 

 line becoming very clearly marked. Meanwhile the granule 

 becomes slightly drawn out into a spindle shape, ^^ith the 

 vesicular material applied along one surface (figs. 18 and 19). 

 In fig. 19 various aspects (oblique, cross, and longitudinal 

 optical sections) of the acrosome at this stage are shown. It 

 will be noted that the acrosome tends to be located on the 

 nuclear membrane at the edge of the chromatic lining. 



It soon becomes evident that the assumption of the spindle 

 form by the acrosome is merely the initial stage in a process 

 of elongation which now progresses rapidly (fig. 20 et seq.). 

 In this elongation one end of the acrosome is temporarily fixed 

 near the anterior pole of the sperm head, the acrosome thus 

 growing backward over the nuclear wall until it projects con- 

 siderably behind the nucleus (figs. '20, 22, 24 to 27). Having 

 reached the stage shown in fig. 27 the acrosome becomes 

 detached anteriorly, and shdes bodily forward until the 

 originally posterior free end becomes applied to the anterior 

 nuclear wall. Steps in this remarkable migration of the 

 acrosome are shown in figs. 27 to 31. E^-entually the posterior 

 tip of the acrosome seems to be attached to the nucleus at the 

 point of insertion of the axial filament (figs. 31 et seq.). The 

 cytoplasm in the head region is at first carried forward with 

 the acrosome (figs. 28 to 33), but in later stages it gradually 

 moves backward (figs. 35 to 38) until the entire head region 



