THE SPERMATJIGENESIS OF DOMESTIC MAMMALS. 365 
Irom the appearance of the chromosomes it is evident that the real 
character of the second division in the case of the horse is simply an ordinary 
mitosis, the accessory chromosome being also divided into two, like the 
ordinary ones. 
V. The Spermatid. 
From what is stated above, it is obvious that one half of the spermatids 
contains eighteen chromosomes, the other half nineteen (eighteen ordinary 
chromosomes plus one accessory). Immediately after the second division, the 
chromosomes begin to break up and the nucleus enters into the resting stage 
(Figs. 58, 59). 
The spermatids in the resting stage contain several chromatin masses 
(Figs. 60, 61, 65) and thus a phenomenon of dimorphism in this stage in 
regard to the existence of the chromatin nucleolus can not be determined. 
GuyEr (’10) has, however, found the chromatin nucleoli in one half of the 
spermatids in man and believes that these nucleoli stand in direct genetic 
continuity with the two eccentric chromosomes seen in the spermatogonia, the 
two chromatin nucleoli and the accessory chromosome of the spermatocytes. 
Similarly WopsEDALEK (13) in the pig and BAcHHUBER (16) in the rabbit 
report the presence of an accessory chromosome in one half the number 
of the spermatids. On the other hand Stevens (11) found that in the 
guinea-pig the spermatids and the spermatozoa are not visibly dimorphie. 
A similar condition was found by Jorpan (711) in the opossum, where he 
says that “the accessory disappears with ordinary chromosomes into the 
nuclear reticulum of the resting spermatid. In later stages the nucleus 
contains a conspicuous cential plasmosome, but all trace of the accessory 
chromosome seems henceforth lost.” 
During the resting stage of the spermatid the centrosome distinctly 
appears near the idiozome (Figs. 61, 63). Fig. 59 shows the telophase of the 
second reduction division in which the centrosome appears at one pole of the 
cell in close contact with the nuclear wall, while the mitosome (Spindelrest- 
körper) and the mitochondria take their position at the opposite side of the 
nucleus in the cytoplasm. 
