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 



ipermatogenesis 

 i 



gepto \fW 



•■iptrmomci/te \^y i 



Sireniory (^ 



iptrm 



\Mulliplicalion Period . 



Croh/lh Period 



Pairing of Chromosomes 

 I Reducwg divnion 



Oogenesis 

 1 



\ 



Oti}ifiio 



',i;r@ ©(V)© 



Soerm- 



@) I) 

 I 



Pnmorij elci/li 



\ I / Y See one/aril cSei^tt 

 iprufn ana lirit 

 '^ fjolar body) 



I V^noture oyum 



ana potor bff^iif 



/ /|' ) fiaXurt »vun 



rulinumbircf ( ^ "^ \ zygote or 

 'fiJlSreS"" \ * - } I'E'.'.'i."' 



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 t^qoical 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. 



