SPERMATOGENESIS. 



231 



The division of the spermatogone results in two cells, of which one retains the 

 position of the parent cell (which it replaces as a new spermatogone destined for a 

 succeeding division), while the other passes inwards, enlarges and becomes a mother 

 cell or primary spermatocyte. This element, conspicuous by reason of its size and 

 large nucleus, undergoes mitotic division and gives rise to two daughter cells, the 

 secondary spermatocytes or prespermatids. The latter almost immediately divide and 

 produce smaller cells, the spermatids, by whose transformation the spermatozoa 

 directly arise. It is important to note that the spermatids contain only one-half of the 

 number (probably twenty-four) of chromosomes normal for the ordinary (somatic) 

 cells of the human body, a like reduction (page 9) occurring during the maturation 

 of the ovum. The spermatids, at first small cells with round nuclei, elongate; their 

 nuclei coincidently become oval and smaller, but rieh in chromatin, and shift to the end 



FIG. 282. Diagram illustrating phases of one complete cycle of spermatogenesis. Sequence of figures 

 shows in detail growth (1-6) anddivision (7-8) of spermatogone; growth and division of primary sper- 

 matocyte (9-19) into secondary spermatocytes; division of latter (20-21) into spermatids (22-24); fusion 

 of these with Sertoli cell to form spermatoblast (25-26); differentiation (27-31) and final liberation (32) of 

 spermatozoa. (After Ebner.) 



of the cell most removed from the lumen of the tubule. The modified spermatids now 

 become closely related to a Sertoli cell, with whose cytoplasm they fuse. The struc- 

 ture thus formed, known as the spermatoblast, consists of an irregular nucleated 

 conical mass of protoplasm, with the inner end of which the radiating clusters of par- 

 tially fused spermatids are blended (Fig. 282). The succeeding changes include the 

 transformation of the elongated nucleus of the spermatid into the head, and of its 

 double centrosome (diplosome) into the neck-granules and (according to some) the 

 axial filament of the spermatozoon, while from the cytoplasm of the spermatid its 

 remaining parts are derived. As the spermatozoa become more and more differ- 

 entiated, they appear as fan-shaped groups in which the heads are always buried 

 within the spermatoblast and the tails directed towards the lumen of the canal. 

 After subsequent separation, the liberated spermatic filaments occupy the centre of 

 the tubule as masses which often occlude the lumen and in which the spermatozoa 

 are disposed in peculiar whorl-like groups. Their completed development, however, 



