114 



TEXTBOOK OF ZOOLOGY 



Oogenesis begins with the primordial germ cell within the ovary. 

 These cells are typically spherical or oval with a prominent nucleus, 

 having the normal number of chromosomes for the somatic cells of 

 the species. This number of chromosomes is known as the diploid 

 number. For purposes of illustration the process will be described 

 for a form whose diploid number of chromosomes is eight. The pri- 

 mordial cell divides by mitosis to form two oogonia. Each of these 

 divides similarly. As is typical of mitotic division, each chromosome 

 divides with the division of the cell. This series of divisions con- 

 stitutes the multiplication period of the maturation process. In some 



Oogenesis 



Primordial _ f^ 



qermcell WC 



Spermatoqemsis 



Primary -(fi\\ 



oocyte \'^(/ 



Secondary f. i v 



oocyte "\ / 



Nature //T 



ovum { f j 



[it. polar body 

 Zr>d. polar body^--'' 



rertili5ed ovum (Zygote) 



Fig. 47. — Maturation of the germ cells. Oogenesis includes the maturation 

 divisions of the female germ cells or ova, and spermatofjenesis is a similar process 

 of division in the development of mature male germ cells or spermatozoa. 



Primordial 

 qerm cell 



J'permafco- 

 ■ gonia 



--Zbromosome 



Primary 

 ipermatocybe 



_ Secondary 

 spermato- 

 cyte 



/(• jSpermatid 



Mature 

 spermato- 

 zoon 



instances each of these cells divides once more. Next, each of these 

 oogonia passes through a growth period without division. During 

 this time the chromosomes in each unite in pairs and fuse together. 

 This fusion is spoken of as synapsis of chromosomes. At the close 

 of this growth each of these cells is called a primary oocyte. Each 

 of these oocytes divides by meiosis, the fused chromosomes dividing 

 as though they were single ones in normal division. This division, 

 therefore, results in cells with half the somatic (diploid) number of 

 chromosomes and is spoken of as the reduction division. The cyto- 

 plasm does not divide equally; nearly all of it goes to one of the 



