132 OUTLINES OF EVOLUTIONARY BIOLOGY 



somatic cells have eight the mature ovum or spermatozoon will 

 have only four, and so on. This reduction of the number of 

 chromosomes (nieiosis) is the essential part of the process of 

 maturation which the animal germ cells undergo, and it is 



Spermalogenesis 

 1 



Spermatogonia 



Multiplication of Epithelial 

 Cells (Spermatogonia and 

 Oogonia) in the Gonad 

 (Test is or Ovary) by ordinary 

 Somatic Mitosis 



Oogenesis 



Oogonia 



Primary 



Secondary 



"ifliiii ^ 



1 / J ) / / / ^fonjugatton of / 



/[//(/(/ \Gametes or .' , 

 (I I 1 fertilization^ f 



^Primary 

 'Oocytes 



^ Secondary i 

 K <?n</ /"c/d/ 1 fiotf/fi 



W^w^ ?J 

 '<* Polar Bodies 



MJ Mature Ova 

 Female Gametes 



Male Gametes 



&J Zygote or Fertilised Ovum 



C^X) (The figures indicate the numbers 

 Segmentation of ;'; ;; oF Chromosomes present) 

 the Zygote by 

 Ordinary Somatic 0$) 

 Mitosis 00 



I 

 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 



