13-2 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 (meiosis) is the essential part of the process of 

 maturation which the animal germ cells undergo, and it is 



Spermalogenesis 

 1 



Spermatogonia ^P 



Oogenesis 



Multiplication of Epithelial 

 Cells ( Spermatogonia and 

 Oogonia) in the Gonad 

 (Test is or Ovary) by ordinary 

 Somatic Mitosis 



? Oogonia 



Primary /J^ 



Spermatocytes >r 



Secondary 

 Sperm 



Spermatozoa 



Synaps/s or Pairing xs! 

 of Che Chromosomes v/ 



Reducing Division 

 f Meiosis ) 



. Primary 



Male Gametes 



Polar 

 Bodies i 



- Conjugation of ' 

 ' 







L Secondary Oocytes 

 4 / and Polar Bodies 



l Mature Ova 

 Polar todies 



Mature Ova 



Female Gametes 



C&J Zygote or Fertilised Ovum 



TT 



Segmentation of 

 the Zygote by 

 Ordinary Somatic 

 Mitosis 



(The Figures indicate the numbers 

 of Chromosomes present) 



FIG. 65. Diagram of Oametogenesis. 



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 



