THE MOSAIC STAGE OF DIFFERENTIATION 255 



We may begin with the conditions in the Anura, which have been 

 very thoroughly investigated.^ The primordial germ-cells arise in 

 the dorsal region of the gut-wall, and then become separated from 

 the rest of the endoderm as a continuous ridge dorsal to the mesen- 

 tery. This ridge later divides into two. In these two genital ridges, 

 the germ cells are mixed with mesenchyme, and overlain by coelo- 

 mic epithelium which becomes slightly thickened. Later the core of 

 the ridges is invaded by the rete tissue, consisting of cords of cells 

 which appear definitely to grow out from the rudiment of the 

 mesonephros. The sexually undifferentiated gonad-rudiment is 

 now completely constituted, and consists of two portions, a peri- 

 pheral cortex composed of coelomic epithelium and primordial 

 germ-cells with associated mesenchyme, and a central medulla 

 derived primarily from the mesonephros. The cortex is broadly 

 homologous with the germinal epithelium of Amniota. 



Sexual differentiation now occurs. In the female the cortex en- 

 larges, its contained germ-cells develop into oogonia and oocytes ; 

 meanwhile the medulla ceases growth and develops into epithelial 

 ovarial sacs. In the male, the rete cords of the medulla continue to 

 proliferate, and are invaded by the germ-cells, which leave the 

 cortex and migrate inwards, then proceeding to differentiate into 

 spermatogonia. Later the rete cords produce, among other struc- 

 tures, the non-germinal portions of the testis tubules. Meanwhile 

 the cortex, after losing its germ-cells, becomes reduced to a thin 

 peritoneal covering. 



The evidence appears conclusive, first, that the type of sexual 

 differentiation of the indifferent gonad-rudiment is normally 

 dependent on its genetic sex-constitution, although, as we shall see 

 later, this can be overridden by other agencies. The case is like that 

 of any other mosaic differentiation, except that the gonad-field has 

 one of two potentialities open to it, according to the sex-chromo- 

 somes which it contains. Secondly, the primordial germ-cells 

 appear to be completely bi-potential as regards sex. What they shall 

 become is determined by local influences emanating from the region 

 in which they come to lie. In the cortex they become female, in the 

 medulla, male. In other words, their differentiation is dependent. 



Temperature exercises a differential effect upon the cortex and 



^ Full references in chapters by Willier and by Witschi in E. Allen, 1932. 



