210 



ESSENTIALS OF ZOOIiOGY 



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 

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

 oogonia passes through a groivth 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 



Oogenesis 



Primordial f^ 



qermcell 'W'i 



Spermatoqenesis 



Primary Yff W 



oocyte \^// 



fi)\\ Primordia] 



Spermato- 

 '-" gonia 



— Chromosome 



7/i\ Primary 



(( ^T sperTT)atocyt<i 



Secondary A/x 



ndcvtz ((^j^ 



oocyte 



"—^ /Sc formation 



1st. polar body 



Zr'd- polar body-"-' 



rertiliied ovum (Zygote) [^ '^ 



Fig. 60. — Maturation of germ cells. Oogenesis includes the maturation divisions 

 in development of the female germ cells or ova, and spermatogenesis is a similar 

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



{ l\ Secondary 

 f ( J spermato- 



cyte 



Mature 



'" spermabo- 

 ^oon 



these oocytes divides by meiotic division, the fused chromosomes 

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

 division, therefore, results in cells with half the sQ^matic (diploid) 

 number of chromosomes and is spoken of as the reduction division. 

 The cytoplasm does not divide equally ; nearly all of it goes to one 

 of the cells in each case. This large cell is called the secondary 

 oocyte and the small one is the first polar body. Each of these cells 

 has four chromosomes. Following this the secondary oocyte divides 

 to form the mature ovum and another polar body. Occasionally the 

 first polar body divides, but none of them have any further signifi- 



