138 GENERAL EMBRYOLOGY 



cell divisions, and these occur simultaneously, while the 

 nuclear and cytoplasmic divisions occur consecutively at a 

 later period, during which these chromosomes do not divide 



again. The number of tetrad groups is thus the same as the 



AA 

 haploid number of chromosomes ( ^ K and the total number of 



elements composing the tetrads is four times the haploid or 

 two times the diploid number (2s). 



The nucleus and cell now enter upon a mitosis in which each 

 tetrad behaves as a typical chromosome. The division and 

 migration of the centrosomes to opposite sides of the nucleus, 

 the formation of the spindle and asters, and other details of 

 this mitosis, have no unusual features and need not detain us. 

 The tetrads, containing all told 2s elements, become arranged 

 about the equator of the spindle and each separates into two 

 pairs of elements called the dyads (Fig. 73, F, G). The groups 

 of dyads then move to opposite poles of the spindle and the cell 

 divides into the two secondary spermatocytes. Since the rest- 

 ing stage is now omitted, the dyads do not dissolve after this 

 division, nor do they divide again in anticipation of the next 

 mitosis the division of the chromatic elements for this cell 

 division has already occurred in the nucleus of the primary 

 spermatocyte, as we have seen. The dyads, containing all told 

 s elements, then move at once to the equator of the new spindle, 

 and each separates into two monads (Fig. 73, 7, /). The two 



groups of monads, each now containing - elements, diverge to 



2i 



opposite poles of the spindle, and the division of the cell 

 (secondary spermatocyte) results in the formation of two sper- 



o 



matids, each with - chromosomes (Fig. 74). Each nucleus 



then reforms into a typical resting condition, and passes through 

 the metamorphosis into the head of the spermatozoon, as 

 described in the preceding chapter. The essential characteris- 

 tic, therefore, of the nucleus of the spermatid and spermatozoon 

 is that, while each of the bivalent chromosomes of the spermatocyte 

 is represented, yet, as the result of the process of reduction 



