164 THE FOUKTH DAY. [CHAP. 



From the upper end of the ureter diverticula are given 

 off at right angles into the intermediate cell-mass. These 

 lengthening and becoming twisted, form the tubuli urini- 

 feri, while the mesoblast around their extremities becomes 

 directly converted into the Malpighian bodies and the 

 capillary network of the kidneys. Corresponding to the 

 relative position of their ducts, the kidney lies above the 

 Wolffian body. At its first appearance it forms an oval 

 body, lying in the upper part of intermediate cell-mass 

 between the Wolffian body and the vertebral column, and 

 is placed rather nearer the median line than the Wolffian 

 body. 



The formation of the kidneys takes place before the end 

 of the seventh day, but they do not become of functional 

 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 separation from these 

 is an occurrence of a purely secondary importance. 



20. Before describing the subsequent fate of the Wolffian 

 and Mullerian 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 inter- 

 mediate 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 distance outwards on either 

 side of the ridge in the cells lining the most median portions 

 of both somatopleure and splanchnopleure. Passing out- 

 wards along these layers, the columnar cells gradually give 

 place to a flat tesselated epithelium. As the ridge con- 

 tinues to increase and project, the columnar character be- 

 comes more and more restricted to cells covering the ridge 

 itself, in 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 undergoes, as 

 we have seen, an involution to form the duct of Muller, and 

 for some little time retains in the immediate neighbourhood 

 of that duct its columnar character (Fig. 51, a'), though 

 eventually losing it. 



