2i6 Vertebrate Embryology 



origin of the muscles of the appendages is 

 independent of the muscle plates. 



The Wolffian body. — During the third day, 

 the mass of mesoblast between the meso- 

 blastic somites and the point of divergence 

 of the somatopleure and the splanchnopleure, 

 the intermediate cell-mass, becomes very 

 prominent, and is covered with a sharply 

 defined layer of epithelial cells (Fig. 54). 

 The intermediate cell-mass is of importance 

 because from it, or in it, are developed the 

 urinary and reproductive organs. The de- 

 velopment of the Wolffian duct has already 

 been described. The origin of the Wolffian 

 body, or embryonic kidney, of the chick will 

 now be briefly outlined. 



It will be recalled that, in the frog, the 

 embryonic or temporary excretory organ was 

 the head-kidney or pronephros ; and that, as 

 the Wolffian body or permanent kidney was 

 developed, the pronephros gradually disap- 

 peared. In the chick, the embryonic kidney 

 is the mesonephros or Wolffian body, and this 

 ceases to function as an excretory organ shortly 

 after hatching, and is replaced, in that capacity, 

 by the m.etanephros or permanent kidney. 

 The pronephros, in the chick, is an extremely 



