390 THE DEVELOPMENT OF THE CHICK 



of the metanephrogcnous 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 (cf. 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 clorsally and at the 

 angle next the aorta. With the subsequent degeneration of the 

 mesonephros, and projection of the kidney into the coelome, 

 its ventral surface acquires a secondary peritoneal capsule. 



III. The Organs of Reproduction 



The gonads are laid down on the median surface, and the 

 ducts on the lateral surface of the Wolffian body, which thus 

 becorries converted into a urinogenital ridge. The composition 

 of the urinogenital ridge is at first the same in all embryos, whether 



