384 THE DEVELOPMENT OF THE CHICK 



II. THE 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 primordial (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 Wolffian 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 csecal 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 



