220 THE FOURTH DAY. [CHAP. 



The formation of the kidneys takes place before the 

 end of the seventh day, but they do not become of func- 

 tional importance till considerably later. 



From their mode of development it clearly follows 

 that the permanent kidneys are merely parts of the 

 same system as the Wolffian bodies, and that their se- 

 paration from these is an occurrence of a purely second- 

 ary importance. 



The generative ridge. Before describing the sub- 

 sequent fate of the Wolffian and Miillerian ducts, it will 

 be necessary to give an account of the formation of the 

 true sexual glands, the ovaries and testes. 



At the first appearance of the projection from the in- 

 termediate cell-mass, which we may now call the genital 

 ridge, a columnar character is not only visible in the 

 layer of cells covering the nascent ridge itself along its 

 whole length, but may also be traced for some little dis- 

 tance outwards on either side of the ridge in the cells 

 lining the most median portions of both somatopleure and 

 splanchnopleure. Passing outwards along these layers, 

 the columnar cells gradually give place to a flat tesse- 

 lated epithelium. As the ridge continues to increase 

 and project, the columnar character becomes more and 

 more restricted to cells covering the ridge itself, over 

 which at the same time it becomes more distinct. On 

 the outer side of the ridge, that is on the side which 

 looks towards the somatopleure, the epithelium under- 

 goes, as we have seen, an involution to form the com- 

 mencement of the duct of Muller, and for some little 

 time retains in the immediate neighbourhood of that 

 duct its columnar character (Fig. 71, a'), though even- 

 tually losing it. 



