MATURATION 



139 



through synapsis, the chromosomes are now only half as numer- 

 ous as in all the other cells of the organism. But while each 

 bivalent chromosome is thus represented, it may be a question 



FIG. 74. Diagram of reduction with tetrad formation in sperm atogenesis. 

 From Wilson, "Cell." The somatic number of chromosomes is supposed to be 

 four. A,B. Division of one of the spermatogonia, showing the full number 

 (four) of chromosomes. C. Primary spermatocyte preparing for division; the 

 chromatin forms two tetrads. D,E,F. First division to form two secondary 

 spermatocytes each of which receives two dyads. G,H. Division of the two 

 secondary spermatocytes to form four spermatids. Each of the latter receives 

 two single chromosomes and a centrosome which persists in the middle-piece of 

 the spermatozoon. 



whether each univalent chromosome is also somehow repre- 

 sented. To this we shall return later. 



But the formation of tetrads is by no means of universal, 

 even of common, occurrence in reducing divisions. Tetrads are 

 commonly found only among the Nematodes, Annelids, and 



