THE SPERMATOGENESIS OF DOMESTIC MAMMALS. 383 
metaphase the behavior of the chromosomes is just the same as that of the 
spermatogonia in the earliest generation, but the size of the chromosomes of 
the penultimate spermatogonia is larger than that of the earliest generation 
(Figs. 37, 38, 29). Fig. 37 exhibits a polar view of the metaphase of the 
penultimate spermatogonium. The chromosomes here are arranged radially 
around the central space, and their number and form are just the same as 
those of the spermatogonia in the earliest generation. The idiozome is situated. 
in close contact to the nuclear wall, but a centrosome can not be demonstrated 
within it. 
III. The Spermatocyte. 
A. THE Grow TH PERIOD. 
The leptotene stage:—The daughter cells resulting from the division of the 
ultimate spermatogonia are the spermatocytes, and these pass gradually to 
the growth period. 
The chromosomes are not fused immediately after the division of the 
ultimate spermatogonia thus it is possible to distinguish individual chromo- 
somes (Figs. 39, 46, 47). The next change is that the nucleus becomes larger, 
while the chromosomes grow more irregular in form (Fig. 47), and no nucleolus 
is formed in any part of the nucleus. 
It is conceivable that in this stage a continuous chromatin spireme is 
not formed, as free ends of certain spiremes may always be distinguished 
(Figs. 48—50). The cytoplasm appears granular, but its precise structure 
as well as the idiozome and the centrosome can not be identified (Figs. 39 
to 45). 
The synaptene stage :—The leptotene threads begin to converge towards 
one side of the nucleus, leaving a clear space on the other side (Figs. 41, 
51). Neither an idiozome nor a ceutrosome can be identified at the converging 
point of the chromosomes, although in the horse the idiozome is situated at 
this point. Finally the spiremes collect together and form a mass, but there 
is no eyidence that the chromosomes become fused and thus lose their 
individuality (Fig. 51). All the appearances of the nuclear structures indicate 
rather that the spiremes are only very closely massed, for the sections always 
