Cells and Their Activities 21 S 



The two cells resulting from this division are known as secondary 

 spermatocytes or spermatocyte II. Again the typical spindle is formed, 

 and the cell rapidly undergoes the second meiotic or maturation divi- 

 sion. In this division, the split chromosomes separate to go to separate 

 cells, and the cells resulting from each secondary spermatocyte will 

 have exactly the same chromosomal content. Not only is the same 

 number retained, but the chromosomes themselves are the same. This 

 is a true eqtiational division, and the resulting cells, the spermatids, have 

 a haploid (N) number of chromosomes. Each primary spermatocyte 

 thus by this series of divisions produces four spermatids. 



By a series of morphological changes, the spermatids become the 

 motile spermatozoa. Each spermatozoan (Fig. 76,5) consists of a head, 

 middle piece, and tail. The head is formed from the nucleus and thus 

 contains the chromatin material derived from the chromosomes ; the mid- 

 dle piece contains the mitochondria and the centrosomes ; the tail is 

 simply an axial filament with the plasma membrane around it. 



Oogenesis. — Oogenesis, or the formation of the egg, involves the 

 same nuclear changes as does spermatogenesis; that is, the primordial 

 germ cell by a series of mitotic divisions produces a large number of 

 oogonia. These develop into primary oocytes, which divide to produce 

 secondary oocytes, and finally the ootid, which has the haploid (N) 

 number of chromosomes. No morphological changes occur in the ootid 

 as did in the spermatid. 



There is, however, one major difference between spermatogenesis 

 and oogenesis: the number and size of the resulting gametes. Each 

 primary spermatocyte produces four small spermatids; each primary 

 oocyte produces but one relatively large ootid or egg and two or three 

 small nonfunctional bodies. 



When the primary oocyte divides to produce the secondary oocytes, 

 the tetrads form near the periphery of the cell, and the resulting division 

 distributes the chromosomes in an equivalent manner, but not the cyto- 

 plasm. This produces two cells of unequal size, one is the larger second- 

 ary oocyte, the other the first polar body. This polar body is chiefly 

 chromatin material. It may or may not divide, but at any rate it 

 rapidly degenerates and disappears. The division of the secondary 

 oocyte likewise results in two cells of unequal size: the larger ootid 

 and another short-lived polar body. The ootid thus has a large amount 

 of cytoplasm and frequently stored food for the benefit of the future 

 embryo. The ootid is also known as the mature ovum. 



