SPERMATOGENESIS 



119 



mitosis so that the spermatocytes eventually formed have 

 each the typical number of four chromosomata. But in the 

 spermatocyte preparing for division the chromatin, instead of 

 dividing into four chromosomes, splits up into eight rod-shaped 

 bodies, united by linin threads into two groups of four, or 

 tetrads. On the formation of the spindle the tetrad groups 



Fig, 24. 



Spermatogenesis in Ascaris megalocephala var. bivalens. A, a. sper- 

 matogonium with four rod-shaped chromosomes. B, a spermatocyte 

 preparing for division. Instead of four chromsomata there are two 

 groups of four, or tetrads. C, first division of the spermatocyte ; 

 two members of each tetrad are passing to opposite poles of the 

 spindle. D, first division complete ; two secondary spermatocytes 

 are formed, each containing two dyads. E, second division of the 

 spermatocytes, showing the halving of the dyads, f, the division 

 completed, resulting in four spermatids, each containing two chro- 

 mosomes. (After Boveri.) 



are arranged at the equator, and two members of each group 

 of four pass to the opposite poles of the spindle. At the close 

 of the division there are two daughter-spermatocytes, each 

 containing two groups of two, or dyads. These two cells 

 immediately divide again ; the dyad groups are arranged at 

 the equators of the spindles as the tetrad groups were in the 

 preceding division, and one member of each of the two dyad 



