576 
J. BRONTE GATENBY. 
As the mitochondria slough down the sperm tail, the mito- 
chondrial sheath becomes thicker and more darkly staining in 
the region just cleared of the chondriosomes, the natural 
inference being that the latter form the sheath. This seems 
supported by such a stage as that drawn in PL 34, fig. 45, at 
X, Y, and Z. 
In the areas where the cells of the male generation are 
densely packed one often finds variations in Nebenkern, 
mitochondria, and nucleus. Without at present going into 
the question of the cause of these variations, whether technical 
or inherent in the snail, it is proposed to describe them. In 
PI. 32, fig. 25, is drawn a spermatocyte near the end of 
growth period. The mitochondria are seen to be small, 
hollow spheres, scattered throughout the cytoplasm in much 
the same way as in the spermatocyte in PI. 33, fig. 32 k 
The Nebenkern elements are much more numerous than in 
the cases already described, where there are generally thirty 
rodlets in the spermatocyte, and from six to twelve in the 
spermatid. In the other spermatocytes, such as that in PL 32, 
fig. 25, the rodlets are always almost completely circular, that 
slight curve or banana shape being here much exaggerated. 
The circular rodlets generally are placed as shown in PL 32, 
fig. 25, but it will be noticed that they are almost invariably 
placed so that their outer, thicker edge lies outermost, and 
their centre is in contact with the archoplasm. This is very 
well seen in PL 32, fig. 24 a. Re-examination of the Neben- 
kern elements in PL 33, figs. 32, 35, and 36 shows that in this 
variety of cell the elements are banana-shaped, and the 
convex surface or back of the rodlet is turned inwards — 
exactly the opposite of what is found in PL 32, figs. 25 or 24 a, 
etc. Now when the spermatocyte breaks into the prophases, 
the cell looks like PL 32, fig. 27. The chromosomes are 
appearing ; the Nebenkern elements have become disposed 
into two groups, one on each side of the nucleus, evidently 
being influenced by the centrosome. Below the nucleus lie 
several other rods, curved so as to form a circle, but with one 
side especially thickened. It is quite characteristic of these 
