1212 



SPLANCHNOLOGY 



close to where the latter opens into the cloaca (Figs. 1115, 1116). The metanephros 

 arises in the intermediate cell mass, caudal to the mesonephros, which it resembles 

 in structure. The diverticulum from the Wolffian duct grows dorsal ward and 

 forward along the posterior abdominal wall, where its blind extremity expands 

 and subsequently divides into several buds, which form the rudiments of the 

 pelvis and calyces of the kidney; by continued growth and subdivision it gives 



Wolffian duct 



Allantois \ Kidney diverticulum 

 Umbilical cord I I I Rectum 



Umbilical vessels 

 Hind-gut 



yotochord 



FIG. 1115. Tail end of human embryo twenty- 

 five to twenty-nine days old. (From model by Keibel.) 



FIG. 1116. Tail end of human embryo thirty-two 

 to thirty-three days old. (From model by Keibel.) 



Ureter 



rise to the collecting tubules of the kidney. The proximal portion of the diver- 

 ticulum becomes the ureter. The secretory tubules are developed from the 

 metanephros, which is moulded over the growing end of the diverticulum from the 

 Wolffian duct. The tubules of the metanephros, unlike those of the pronephros 

 and mesonephros, do not open into the Wolffian duct. One end expands to form 

 a glomerulus, while the rest of the tubule rapidly elongates to form the convoluted 

 and straight tubules, the loops of Henle, and the connecting tubules; these last 



join and establish communications 

 with the collecting tubules derived 

 from the ultimate ramifications of the 

 diverticulum from the Wolffian duct. 

 The mesoderm around the tubules be- 

 comes condensed to form the connec- 

 tive tissue of the kidney. The ureter 

 opens at first into the hind-end of the 

 Wolffian duct; after the sixth week it 

 separates from the Wolffian duct, and 

 opens independently into the part of 

 the cloaca which ultimately becomes 

 the bladder (Figs. 1117, 1118). 



The secretory tubules of the kid- 

 ney become arranged into pyramidal 

 masses or lobules, and the lobulated 

 condition of the kidneys exists for 

 some time after birth, while traces of 

 it may be found even in the adult. The kidney of the ox and many other animals 

 remains lobulated throughout life. 



The Urinary Bladder. The bladder is formed partly from the entodermal 

 cloaca and partly from the ends of the Wolffian ducts; the allantois takes no share 

 in its formation. After the separation of the rectum from the dorsal part of the 

 cloaca (p. 1109), the ventral part becomes subdivided into three portions: (1) an 

 anterior vesico-urethral portion, continuous with the allantois into this portion the 

 Wolffian ducts open; (2) an intermediate narrow channel, the pelvic portion; and 



Glans penii 



Urethra 



Vertebral column 



Fia. 1117. Tail end of human embryo; from eight and a 

 half to nine weeks old. (From model by Keibel.) 



