130 



NES OF EVOLUTIONARY BIOLOGY 



have eight the mature ovum or spermatozoon will 

 A four, and so on. This reduction of the number of 

 /omes (meiosis) is the essential part of the process of 

 /ation which the animal germ cells undergo, and it is 



Spermalogenes 

 I 



Spermatogonia Q , 



Oogenesis 



Multiplication of Epithelial 

 Cells ( Spermatogonia and 

 Oogonia) in the Gonad 

 (Test is or Ovary) by ordinary 

 Somatic Mitosis' 



? Oogonfo 



Male Gametes 



) Zygote or Fertilized Ovum 



C*L\jp (The Figures indicate the numbers 

 Segmentation of ;'( ;; of Chromosomes present) 

 the Zygote by M } 

 Ordinary Somatic @@ 

 Mitosis ?) 





FIG. 65. Diagram of Gametogenesis. 



effected by a modification of the mitotic nuclear division, in 

 which the chromosomes are separated into two groups, half the 

 total number of entire chromosomes going into one daughter 

 cell and half into the other. 



In a typical case of spermatogenesis the first stage is the 



