THE DEVELOPMENT OF THE UROGEXITAL SYSTEM. 



393 



the process by which the proximal ends of some of the blood vessels come to be 

 included in the wall of the heart (p. 235). The proximal ends of the tubules 

 become wider, the pelvis swells out, and the walls of the tubules become in- 

 cluded in the wall of the pelvis. In certain parts of the pelvic wall this process 

 goes on until deep bays the calyces are formed, into which a large number of 

 tubules open. In the other parts of the wall the process does not go so far, thus 

 leaving promontories the renal papilla upon which larger tubules or papil- 

 lary ducts open. The adult renal pelvis thus consists of the primitive pelvis plus 

 the proximal ends of the straight tubules. 



Metanephric I 

 blastema | 



Primitive 

 renal pelvis 



Ureter 



Mesonephric duct 

 Intestine 



. Bladder 



FIG. 351. From a transverse section of a human embryo at the beginning of the 5th week. 

 The plane of the section is indicated in Fig. 349. Schreiner. 



The Convoluted Renal Tubules and Glomeruli. As stated above, 

 the metanephric blastema or nephrogenic tissue surrounds the renal pelvis 

 and the straight tubules. It represents a condensation of the mesenchyme and is 

 destined to give rise to the convoluted tubules and glomeruli. The cells of the 

 blastema in the region of the ampullae of the terminal straight tubules acquire 

 an epithelial character and become arranged in solid masses (Fig. 353). Each 

 mass unites with an ampulla and acquires a lumen, which becomes continuous 

 with the lumen of the straight tubule, then elongates and forms an S-shaped 

 structure (Figs. 354 and 355). The loop of the S nearer the straight tubules 

 elongates still more and grows toward the pelvis, parallel with the straight 



