390 THE DEVELOPMENT OF THE CHICK 
of the metanephrogenous tissue takes place in essentially the 
same manner as the formation of the mesonephric tubules. The 
first stages may be found in seven and eight-day chicks in the 
portion of the kidney behind the umbilical arteries. The inner 
zone tissue begins to arrange itself in the form of minute balls 
of cells in immediate contact with the secreting tubules; a small 
lumen then arises within the ball, transforming it into a thick- 
walled epithelial vesicle with radially arranged cells. The vesicle 
then elongates away from the collecting tubule and gradually 
takes on an S-shape. The distal end of the S becomes con- 
verted into a renal corpuscle as illustrated in Figure 225, 
and the proximal end fuses with the wall of the collecting tubule; 
an opening is then formed between the two. 
On the eleventh day of incubation, secreting tubules are thus 
formed throughout the entire length of the kidney; but the histo- 
logical structure does not yet give the effect of an actively secret- 
ing gland, although degeneration of the mesonephros has already 
begun. The full development of the nephric tubules in the 
chick has not been studied. 
At all stages in its development the kidney substance is 
separated from the mesonephros by a distinct layer of undiffer- 
entiated mesenchyme, which is, however, at certain times ex- 
tremely thin. But there is no evidence that at any time elements 
of the mesonephros, e.g., undifferentiated nephrogenous tissue, 
extend up into the metanephric primordium which so closely 
overlies it (ef. Figs. 221 and 222). 
The kidney is entirely retroperitoneal in its formation, and 
its primary capsule is established by differentiation of the periph- 
ery of the outer zone. This may be seen in process at eleven 
days (Fig. 222): the primary capsule is definitely established on 
its median and lateral sides; but is defective dorsally and at the 
angle next the aorta. With the subsequent degeneration of the 
mesonephros, and projection of the kidney into the ccelome, 
its ventral surface acquires a secondary peritoneal capsule. 
III]. Tur OrGans or REPRODUCTION 
The gonads are laid down on the median surface, and the 
ducts on the lateral surface of the Wolffian body, which thus 
becomes converted into a urinogenital ridge. The composition 
of the urinogenital ridge is at first the same in all embryos, whether 
