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 along 
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. 
