362 The Spermatogenesis of Bufo Lentiginosus 
before the division of the cell is completed. The second mitosis is longi- 
tudinal, completing the division that is begun in some of the chromo- 
somes at the stage of Fig. 31, and in others not until the late anaphase 
of the first mitosis (Fig. 51). 
THE SPERMATIDS AND SPERMATOZOA. 
The nucleus of the young spermatid occupies a very eccentric posi- 
tion, lying in the portion of the cell that is to become the head of the 
spermatozoon (Fig. 58). At first the chromatin is distributed through- 
out the nucleus in the form of angular shaped blocks connected by fine 
linin threads (Fig. 58); later it is spread out in an irregular reticulum 
(Figs. 59-65). During the development of the spermatid the nucleus 
loses its rounded shape and becomes greatly elongated, finally appearing 
in the mature spermatozoon as a cylindrical, perfectly homogeneous body 
that has great affinity for all chromatin stains. 
The centrosome is always found in the part of the cell containing the 
greatest amount of cytoplasm. It is not surrounded by a granular at- 
traction-sphere as in the resting stages of the preceding generations, but 
is inclosed in a clear, round or oval vesicle which is sharply marked 
off from the surrounding cytoplasm (Figs. 58, 59, ete.). Whether the 
substance of this vesicle is derived from the attraction-sphere of the 
spermatocytes I have not been able to determine. The attraction-sphere 
disappears in the prophase of the first maturation mitosis and I have 
found no traces of it in later stages, unless, indeed, the radiation found 
for a short time around the centrosomes as the second maturation spindle 
is forming is derived from the substance of the attraction-sphere. 
In the early stages of the development of the spermatids, the vesicle 
inclosing the centrosome may lie freely in the cytoplasm (Fig. 60), or it 
may be in contact with the nuclear membrane (Fig. 58) ; in the latter 
case it marks the region of the nucleus that is to become the posterior 
end of the sperm-head, as the vesicle itself is the anlage of the middle- 
piece of the spermatozoén. The centrosome divides soon after the stage 
shown in Fig. 58, and in favorable preparations one can see that the 
two centrosomes are connected by a very fine, thread-like fibre (Figs. 
59, 60). As the centrosomes move apart, the vesicle inclosing them 
elongates and later it becomes much flattened where it presses against 
the nuclear membrane (Figs. 61, 62, etc.), The middle-piece anlage 
appears perfectly transparent during all stages of its development and 
shows not the slightest affinity for either hematoxylin or safranin stain. 
In the mature spermatozoén, the middle-piece has the same diameter as 
