300 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



Maturation of the egg-cell. — As regards the behavior of the 

 chromosomes the maturation of the ovum parallels that of the sperm- 

 cell. There are not so many primordial germ-cells formed and only 

 one out of four of the ultimate cells becomes a functional egg. As in 

 maturation of the sperm-cell there is a growth period in which oogonia 

 enlarge to become primary oocytes (Fig. 48, b). In each primary 



Spermatogenesis 



Oogenesis 



i 



gofiia \^'^ 



MulltpliCQlion Period . 



t \ 



J>/Jm9rv.. fAt^ 1 



ipermocpc i/te V n^ »/ ' 



CroMth period 



Pairing of Chromosomes 



©•' '% 



\ } Re<^ucing division 



Secanaory ( ^ 

 J per mot o- ' " 

 ei/ttj 



Sperm f <' 



Saerm 

 eRozoa 





Obgonio 



.-■■ s 



Primori^ C80/l^^ 



Seconc/ari/ odcytt 

 fpyufTi and ftrsl 

 ^ polar bo&jf) 



/loture ofufoy , 

 anc/ ptior bt^itj. 



©nature oyum\ 



full number of 

 Chromoiomti 

 restored 



Z If gate or 

 Jeriiliztd 

 ovum 



Fig. 49. — Diagram showing the parallel between maturation of the sperm- 

 cell and maturation of the ovum. {From Guyer.) 



oocyte as in the primary spermatocyte the chromosomes pair and two 

 rapidly succeeding divisions follow in one of which the typical numeri- 

 cal reduction in the chromosomes occurs. A peculiarity in the 

 maturation of the ovum is that there is a very unequal division in 

 the cytoplasm in cell-division so that three of the resulting cells 

 usually termed polar bodies are very small and appear like minute 

 buds on the side of the fourth or egg-cell proper. 



