1 8 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



is reduced in the same manner as in the spermatocyte, so that the ripe 

 ovum and each polar cell contain one-half the number of chromosomes 

 found in the immature ovum or primary oocyte. 



The primitive female germ cells, from which new ova are produced 

 by cell division, are called oogonia and their daughter cells after a period 

 of growth within the ovary are the primary oiicytes, comparable to the 

 primary spermatocytes of the male (Fig. 12). During maturation the 

 ovum and first polocyte are termed secondary oiicytes (comparable to 

 secondary spermatocytes) ; the mature ovum and second polocyte, with 



Spermatogonium 



B 



Oogonium 



Proliferation 

 period 



Spermatocyte I 

 Spermatocyte 2 



Spermatids 

 Spermatozoa 



Oocyte 2 (ovum 

 and polocyte i) 



Ovum and 

 three polo- 

 cytes 



Growth 

 period 



Maturation 

 period 



Transforma- 

 tion period oj 

 spermatozoa 



1234 1234 



FIG. 12. Diagrams of maturation in spermatogenesis and oogenesis (Boveri). 



the daughter cells of the first polocyte, are comparable to the spermatids. 

 Each spermatid, however, may form a mature spermatozoon, but only 

 one of the four daughter cells of the primary oocyte becomes a mature 

 ovum. The ovum develops at the expense of the three polocytes which 

 are abortive and degenerate eventually, though it has been shown that in 

 the ova of some insects the polar cell may be fertilized and segment several 

 times like a normal ovum. In most animals, the actual division of the 

 first polocyte into two daughter cells is suppressed. The nucleus of the 

 ovum after maturation is known as the female pronudeus. 



Maturation of the Mouse Ovum. Typical maturation stages may be 

 studied in the easily obtained ova of the mouse (Long and Mark, Carnegie 

 Inst. Publ. No. 142). The first polocyte is formed while the ovum is 



