384 THE DEVELOPMENT OF THE CHICK 
Il. Tue DEVELOPMENT OF THE METANEPHROS OR PERMANENT 
KIDNEY 
The metanephros or permanent kidney supplants the meso- 
nephros in the course of development. It is derived from two 
distinct embryonic primordia: (1) the nephrogenous tissue of 
the two or three posterior somites of the trunk (31 or 32 to 33), 
which furnish the material out of which the renal corpuscles 
and secreting tubules develop; and (2) a diverticulum of the 
posterior portion of the Wolffian duct (Fig. 219), which develops 
by branching into the collecting tubules and definitive ureter. 
The development of the kidney takes place in a mass of mesen- 
chyme, known as the outer zone of the metanephrogenous tissue, 
that furnishes the capsule and connective tissue elements of 
the definitive kidney, in which also the vascular supply is developed 
(Figs. 221 and 222). The cortical tubules of the kidney are 
thus derived mainly from the nephrogenous tissue, and the medul- 
lary tubules and ureter from the metanephric diverticulum. 
Thus the definitive kidney is analogous in mode of develop- 
ment to the mesonephros, and is best interpreted as its serial 
homologue. This point of view may be regarded as definitely 
established by the work of Schreiner, to which the reader is re- 
ferred for a full account of the history of the subject. 
The metanephric diverticulum, or primordium of the ureter 
and collecting tubules, arises about the end of the fourth day as 
a rather broad diverticulum of the Wolfhan duct at the convexity 
of its terminal bend to the cloaca (Fig. 219). It grows out 
dorsally, forming a little sac, which, however, soon begins to grow 
forward median to the posterior cardinal vein and dorsal to the 
mesonephros (Fig. 224); by the end of the fifth day its anterior 
end has reached the level of the czecal appendages of the intes- 
tine, and on the eighth day its anterior end has reached its defin- 
itive position at the level of the vena cava inferior, near to the 
anterior end of the mesonephros (twenty-first definitive somite or 
twenty-fifth of the entire series; cf. Fig. 150). 
It should be noted that the metanephric diverticulum is similar 
in its mode of origin to the so-called mesonephric ureters. It 
may in fact be regarded as the posterior member of this series, 
but it is separated from those that form the collecting tubules of 
the mesonephros by at least two somites in which no diverticula 
