DAVIS: SPERMATOGENESIS. 115 



are chiefly due to an elongation and flattening of the nucleus, while 

 at the same time the Nebenkern extends back for some distance alone: 

 the sides of the nucleus and goes through a complicated metamorpho- 

 sis. In Figure 208 the Nebenkern is seen to form a deeply staining cap 

 closely applied to the anterior end of the somewhat flattened nucleus, 

 the sides of which it also envelops for a greater or less distance. Fig- 

 ure 209 shows little change except that the nucleus is more elongated. 

 Figure 210 represents a later stage, in which the nucleus has become 

 still more elongated and flattened so that it may be compared to a 

 paddle, the tail corresponding to the handle. Each centrosome has 

 divided, so that four deeply staining granules can be seen at the end 

 of the axial filament. However, the most striking change has taken 

 place in the anterior cap derived from the Nebenkern, which becomes 

 converted into the acrosome. It no longer stains uniformly, but 

 appears as an elongate deeply staining body on each side of the anterior 

 end of the nucleus. The median portion of the cap stains only slightly 

 or not at all with hematoxylin, except along its anterior margin, where 

 there are two rows of minute deeply staining granules, one row lying 

 slightly posterior to the other. Figures 211 and 212 represent a still 

 later stage when the nucleus takes a nearly uniform gray stain, but 

 appears black when viewed from the side, owing to its greater thickness 

 in that direction. Both nucleus and acrosome appear considerably 

 larger as a result of their having become much more flattened. The 

 granules at the anterior end of the acrosome are much more conspicu- 

 ous than formerly and can be seen to be arranged in the form of an 

 ellipse the anterior and posterior sides of which lie at slightly different 

 levels. Figures 211 and 212 give a much better idea of the structure 

 than can be conveyed by an extended description. 



The cell wall can still be distinguished surrounding the nucleus but, 

 as in Dissosteira, I have been unable to find any cytoplasm in this 

 region, although it forms a distinct envelope around the axial filament. 



This is the latest stage in the metamorphosis which I have been able 

 to find in my preparations; but the metamorphosis is evidently nearly 

 completed. 



As in Dissosteira, there are undoubtedly two varieties of sperma- 

 tozoa with respect to their chromatin contents; one half of them con- 

 taining the monosome while the other half lack this element; the two 

 types show, however, no external differences. 



