SPERMATOGENESIS 



119 



The heterotypic or meiotic nuclear mitoses which accompany 

 the first division of the spermatocyte, and the division of the 

 ovum forming the first polar body, are of great interest because 

 they result in the reduction of the chromosomes to half the 

 normal number. Though their main features are fairly well 

 known in the frog, the whole process has not been so thoroughly 



Fig. 24. 



A, Spermatozoa of K ana esciilenta. B, Spermatozoa of Rana 

 fusca (after Leydig). tup, middle piece. 



worked out in this as it has been in some other animals, and 

 the following account, while applicable to the frog, must be 

 understood to be a general description of the nuclear changes 

 as observed in animals and plants. To begin with the 

 spermatozoon. The spermatogonia, throughout their repeated 

 divisions, exhibit the same number of chromosomes as the 

 tissue-cells of the body, this number in the frog being twenty- 

 four. In the first or meiotic division of the spermatocyte this 

 number is obscured, and the behaviour of the nucleus is from 

 the first somewhat different from that observed in ordinary 

 cell divisions. Some of the details of the process are shown 

 in fig. 25, A-F. The nucleus is situated excentrically in the 

 cell-body so that one side of the latter appears to be richer in 

 protoplasm than the other. On this side, overlying the nucleus 

 like a cap, is the denser and more granular centrosphere con- 

 taining the centrosomes. The nucleus about to enter upon 

 the meiotic division is more than usually chromatic. Much of 

 the chromatin is distributed in the form of scattered granules 



