388 



THE DEVELOPMENT OF THE CHICK 



of the cardinal vein, and is in contact, on its median face, with 

 the proper nephrogenous tissue (inner zone); the latter shades 

 into the outer zone, the cells of which are arranged concentrically 

 with reference to the other parts. The relations subsequently 

 established may be summarized in a few w^ords; the inner zone 

 of tissue grows and branches pari passu with the growth and 

 branching of the metanephric diverticulum, so that the termina- 

 tion of every collecting tubule is accompanied by a portion of 



Fig. 224. — Transverse section through the 



ureter and metanephrogenous tissue of a 



o-day chick. 



A. umb., Umbilical artery. Coel., Coelome. 



M's't., Mesentery, n. t. i. z., Inner zone of the 



nephrogenous tissue, n. t. o. z., Outer zone of 



the nephrogenous tissue. Ur., Ureter. V. c. p., 



Posterior cardinal vein. W. D., Wolffian duct. 



the inner zone, which is, however, always distinct from it. This 

 conclusion is established by the fact that from the start the two 

 elements, collecting tubules and inner zone, are distinct and 

 may be traced continuously through every stage. The OTiter 

 zone differentiates in advance of the two more essential con- 

 stituents at all stages, and thus forms a rather thick investment 

 for them. 



The formation of the secreting tubules from the inner zone 



