6 THE EGGS OF MAMMALS 



bryonic life through sexual maturity. The second is con- 

 cerned with varying the conditions of ovarian growth by 

 experimental means and deducing from the derived data 

 the nature of the factors concerned in the production of 

 functional eggs. We shall assume that these two types of 

 observations are distinct, and consider them separately as: 

 (1) the morphogenesis of egg cells and (2) the experimental 

 investigation of the growth of egg cells. 



The Morphogenesis of Egg Cells 



Thanks to the Weismannian controversy we have avail- 

 able a fairly detailed description of oogenesis in embryonic 

 life. It is unnecessary here to enter into a detailed descrip- 

 tion of the embryogeny of the mammalian ovary (see 

 Jenkinson, 1913, de Winiwarter, 1901, de Winiwarter et 

 Sainmont, 1909, Brambell, 1927 and esp. 1930). Our inter- 

 est lies in the so-called ^'primordial" germ cells of the em- 

 bryo, since it is to these cells that a number of observers 

 trace the origin of the definitive ova. 



The general opinion seems to be that large wandering 

 cells originate from the entoderm of the gut before or at 

 the time of the formation of the genital ridges (Nussbaum, 

 1880; Fuss, 1911, 1913). These primordial germ cells migrate 

 to the gonad site and enter the genital ridges. The ridges 

 are first seen as thickenings of the peritoneal epithelium 

 between the base of the mesentery and the Wolffian duct 

 on the ventral side of the developing mesonephros. The 

 thickened peritoneal epithelium becomes the germinal epi- 

 thelium and the primordial germ cells complete their migra- 

 tion when they become arranged beneath this epithelium 

 which then proliferates medullary tissue into the germ cells. 

 The underlying mesenchyme forms connective tissue trabec- 

 ulae in the medulla and also the primitive tunica albuginea 

 which separates the medulla from the germinal epithelium. 



There are among investigators various opinions about the 

 role of the primordial germ cells. A number maintain that 

 these are the only germ cell precursors. The increase in 



