THE URIXOGEXITAL SYSTEM 387 



omous manner, and it is from them that the collecting tubules 

 of the kidney arise; the posterior unbranched portion of the meta- 

 nephric diverticulum represents the definitive ureter. 



The following data concerning these branches should be noted: 



(1) the first ones are formed from the posterior portion of the 

 metanephric diverticulum, and the process progresses in an 

 anterior direction. This is the reverse direction of the usual order 

 of embryonic differentiation, but the reason for the order is the 

 same, viz., that differentiation begins in the first formed parts. 



(2) A posterior, smaller group of collecting tubules is separated 

 at first by an unbranched portion of the ureter from an anterior 

 larger group (Fig. 223). The unbranched region corresponds to 

 the position of the umbilical arteries which cross here. (3) During 

 the fifth and sixth days the terminal portion of the Wolffian 

 duct common to both mesonephros and metanephros is gradually 

 drawn into the cloaca, and thus the ureter obtains an opening 

 into the cloaca independent of the Wolffian duct and posterior 

 to it (Fig. 223). 



The Nephrogenous Tissue of the Metanephros. The nephro- 

 genous tissue of the thirty-first, thirty-second, and thirty-third 

 somites is at first continuous with the mesonephros (Figs. 218 

 and 219), but on the fourth and fifth days that portion situated 

 immediately behind the mesonephros degenerates, thus leading 

 to a complete separation of the most posterior portion situated 

 in the neighborhood of the metanephric diverticulum. This con- 

 stitutes the metanephrogenous tissue proper (inner zone). It is 

 important to understand thoroughly its relations to the metane- 

 phric diverticulum. This is indicated in Fig. 219, which repre- 

 sents a graphic reconstruction of these parts in a duck embryo 

 of 50 somites. It will be seen that the metanephrogenous tissue 

 covers nearly the entire metanephric diverticulum; a transverse 

 section (Fig. 224) shows that it lies on its median side. The 

 outer dotted line (Fig. 219) gives the contour of a dense portion 

 of mesenchyme related to the diverticulum and nephrogenous 

 tissue pro]:)er. In section this forms a rather ill-defined area 

 shading into the nephrogenous tissue on the one hand and into 

 the surrounding mesenchyme on the other. 



Fig. 224 shows the relations of the three constituent elements 

 of the kidney at the end of the fifth day, as seen in a transverse 

 section. The metanej^hric diverticulum lies on the median side 



