VII. J THE PERMANENT KIDNEY. 219 



transverse section lies dorsal to the original duct, and 

 the blind end of which points forwards, that is, towards 

 the head of the chick. This is the duct of the perma- 

 nent kidney or ureter. At first the ureter and the 

 Wolffian duct open by a common trunk into the cloaca, 

 but this state of things lasts for a short time only, and 

 by the sixth day the two ducts have independent open- 

 ings. 



The ureter thus beginning as an outgrowth from 

 the Wolffian duct grows forwards, and extends along 

 the outer side of a mass of mesoblastic tissue which 

 lies mainly behind, but somewhat overlaps the dorsal 

 aspect of, the Wolffian body. 



This mass of mesoblastic cells may be called the 

 metanephric blastema. It is derived from the interme- 

 diate cell-mass of the region reaching from about the 

 thirty-first to the thirty-fourth somite. It is at first 

 continuous with, and indistinguishable in structure 

 from, the portion of the intermediate cell-mass of the 

 region immediately in front of it, which breaks up into 

 Wolffian tubules. The metanephric blastema remains 

 however quite passive during the formation of the 

 Wolffian tubules in the adjoining blastema; and on the 

 formation of the ureter breaks off from the Wolffian 

 body in front, and, growing forwards and dorsalwards, 

 becomes connected with the inner side of the ureter 

 in the position just described. 



In the subsequent development of the kidney col- 

 lecting tubes grow out from the ureter, and become 

 continuous with masses of cells of the metanephric 

 blastema, which then differentiate themselves into the 

 kidney tubules. 



