Helen Dean King 363 
the sperm-head and forms a well marked division between the head and 
the tail (Fig. 71, M. P.). 
One centrosome, which for convenience I shall call the “ inner” 
centrosome, moves to the edge of the vesicle in contact with the nucleus 
and for a time appears as a small rounded projection from the middle 
of the posterior nuclear wall (Figs. 62, 63, 65). Later this centrosome 
becomes imbedded in the deeply staining substance of the sperm-head ; 
whether it aids in the formation of the middle-piece, as Meves finds is 
the case in the spermatids of Salamandra, I have been unable to deter- 
mine. The “outer” centrosome, still keeping its connection with the 
inner centrosome, moves to the posterior end of the vesicle and from 
it a fibre grows out that later forms the axial-filament of the tail (Figs. 
61-63, etc.). In the spermatids of Salamandra and of Amphiuma, the 
outer centrosome becomes ring-shaped and later divides into two parts: 
one part remains at the posterior end of the middle-piece; the other 
part migrates down the axial-filament to the beginning of the end- 
piece of the tail. Owing to the fact that the spermatids in the testis of 
Bufo are very much smaller than those of the Urodela and that the 
posterior region of the cell shows little affinity for either plasma or 
chromatin stains, it is very difficult to follow the history of the outer 
centrosome. As far as I have been able to determine, this centrosome 
never divides, but remains at the posterior end of the middle-piece 
where it soon becomes disc shaped (Figs. 66-68), and later flattens con- 
siderably (Figs. 69-70). In favorable preparations of the mature sperm- 
atozoon, this centrosome appears as a more deeply staining mid-portion 
of the posterior border of the middle-piece; in many cases it cannot be 
found at all. 
In the young spermatid, the acroblast appears round or oval and 
homogeneous as in the earlier stages. It is always found in the posterior 
region of the cell, usually lying some distance from the centrosome from 
which it is readily distinguished on account of its larger size (Fig. 58). 
During an early period in the development of the spermatid, the acro- 
blast. again divides, and occasionally a cell is found containing both of 
the centrosomes and two acroblasts which are considerably smaller than 
those found in the spermatocytes (Fig. 63). One of the acroblasts re- 
mains in the posterior part of the cell (Figs. 66, 68), and as the tail 
forms it stains less intensely than before and subsequently disappears. 
The other acroblast moves gradually towards the anterior end of the 
spermatid, as shown in Figs. 65-67. Transverse sections of the sperm- 
atids will occasionally show this acroblast lying just outside of the 
