190 THE THIRD DAY. [CHAP. 



spoken of as the intermediate cell-mass, is at first indis- 

 tinguishable from the cells lining the inner end of the 

 body cavity ; but on the third day, a special peritoneal 

 lining of epithelioid cells is developed which is more or 

 less sharply marked off from the adjoining part of the 

 intermediate cell-mass. This latter now also passes 

 without any very sharp line of demarcation into the 

 mesoblastic somite itself; and as the folding in of the 

 side wall progresses, the mass of cells in this position 

 increases in size and grows in between the notochord 

 and the hypoblast, but does not accumulate to a suffi- 

 cient extent to separate them widely until the end of 

 the third or beginning of the fourth day. 



The fusion between the intermediate cell-mass and the inner 

 portions of the somites becomes so complete on the third day 

 that it is almost impossible to say which of the cells in the 

 neighbourhood of the notochord are derived from the somites 

 and which form the intermediate cell-mass. It seems almost 

 certain however that the cells which form the immediate invest- 

 ment of the notochord really belong to the somites. 



The intermediate cell-mass is of special importance 

 to the embryologist, in that the excretory and generative 

 systems are developed from it. 



We have already described (p. 106) the development 

 of the Wolffian duct, and we have now to deal with the 

 Wolffian body which is, as the reader has no doubt 

 gathered, the embryonic excretory organ. 



The structure of the fully developed Wolffian body 

 is fundamentally similar to that of the permanent kid- 

 neys, and consists essentially of convoluted tubules, 

 commencing in Malpighian bodies with vascular glome- 

 ruli, and opening into the duct. 



