The Spermatogenesis of Aplopus mayeri. 25 
into this cytoplasmic fin. It is attached to the nuclear wall by a distinct 
chromatic granule, probably a centrosome. Up to this point no structure 
could be definitely decided upon as a centrosome in any of the mitoses, 
and it is only rarely that even an indication of an aster can be observed. 
However, the spindle fibers are always distinct and come to a definite 
point at the poles. Were the centrosome not here pointed out by the 
attached axial filament it would very probably escape notice among the 
various minute chromatic granules of the peripheral zone of the nucleus. 
The nucleus at this stage frequently shows a polar cap of a material that 
stains intensely in iron hematoxylin. This structure is not stained with 
methyl green or thionin and probably represents the head-cap or acrosome 
of the adult spermatozoon (figs. 136 and 137). 
The axial filament now enlarges proximally (fig. 127). While this 
structure elongates distally and sends a slender thread into the long cyto- 
plasmic tail, it differentiates into a proximal stout neck definitely marked 
off from the distal filament, and represents the future middle-piece of the 
spermatozoon. Successive stages with and without the accessory chromo- 
some are shown in figures 128, 129, 130, and 131. The mass of cyto- 
plasm surroundng the axial fiber subsequently becomes the cytoplasmic 
fin of the tail, spirally arranged about the filament (fig. 136). 
Thus far in the metamorphosis the nucleus has remained approximately 
spherical and is surrounded by a thin cytoplasmic envelope. Presently the 
cytoplasm disappears, the nucleus becomes oval in shape, and the proximal 
end of the middle piece widens and flares so as to form a concavity to 
receive the nuclear convexity. The head-cap is conspicuous and the acces- 
sory chromosome retains its compact spherical shape. Later stages show 
processes of disintegration by fragmentation and karyolysis (figs. 133 and 
134) and its final disappearance (fig. 135). The spermatozoon undergoes 
still further changes of form, until in its final stage the nucleus is com- 
paratively small and the middle-piece large. The latter has typically a 
cigar-shape and the nucleus is approximately spherical, with a depressed 
cone-shaped head-cap (fig. 136). 
The mature spermatozoon presents a very strange phenomenon in re- 
gard to its staining reaction to recognized selective chromatin stains. 
Figure 136 shows a spermatozoon stained in iron hematoxylin; figure 137 
shows a spermatozoon of similar age stained with methyl green (or thionin). 
It will be observed that the chromatic portions are exactly reversed as in- 
terpreted by the two stains. The portion picked out by the methyl green 
corresponds with the nucleus (head) of other spermatozoa, and since this 
is a very selective chromatin stain, it probably definitely marks the true 
limits of the nucleus. The reaction obtained with the iron-hematoxylin 
stain, however, yields a very happy result in that it permits the observation 
of the disintegrating accessory chromosome. Figures 138 and 139 show 
