THE URINOGENITAL 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 1 (Mig. 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 proper. 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 metanephric diverticulum lies on the median side 
