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 words; the inner zone 
of tissue grows and branches part 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 
mete te 
y Cru 
ON 
Fic. 224.— Transverse section through the 
ureter and metanephrogenous tissue of a 
5-day chick. 
A.umb., Umbilical artery. Coel., Coelome. 
M’s’t., Mesentery. n.t.i.z., Inner zone of the 
nephrogenous tissue. n. t. 0. 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 outer 
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 
