358 OUTLINES OF CHORD ATE DEVELOPMENT 



or seven tubules are formed in each segment; all these tubules 

 develop similarly. No nephrostomes, or coelomic connections, 

 are formed save in the four or five most anterior tubules, which 

 are themselves transitory structures. The formation of the 

 mesonephric tubules is completed during the fourth and fifth 

 days, when they begin to elongate rapidly. During the next 

 three or four days they become convoluted and form altogether 

 a large mass, sometimes known as the Wolffian body, project- 

 ing from the dorsal body wall. The tubules of each segment 

 open into a common dilation of the Wolffian duct, distin- 

 guished as the collecting tubule. 



The mesonephros becomes very vascular through the forma- 

 tion of abundant sinuses from the accompanying posterior 

 cardinal veins, which are its afferent vessels. The walls of 

 these sinuses are in direct contact with the tubules. The blood 

 collects along the ventral side of the mesonephros in the so- 

 called subcardinal veins, which connect, as we have seen, with 

 the inferior vena cava. The mesonephros begins to degenerate 

 the tenth or eleventh day, and by the time of hatching it has 

 completely disappeared, save in so far as parts of it remain con- 

 nected or associated with the reproductive system. 



C. THE METANEPHROS 



The metanephros is the permanent kidney of the adult, and it 

 also functions probably, together with the mesonephros, during 

 the latter part of embryonic life. Metanephric structures 

 appear toward the end of the fourth day as outgrowths from 

 each mesonephric or Wolffian duct, just as this turns to enter 

 the cloaca. Each outgrowth becomes a sac and then a tube, 

 turning anteriorly and rapidly growing forward along the inner 

 side of the posterior cardinal vein, and finally extending anteri- 

 orly, above the mesonephros, as far as the twenty-fifth somite. 

 This tube is the rudiment of the ureter and collecting tubules 

 of the metanephros; the latter are formed as the result of a com- 

 plicated system of branches of the original duct as it grows 

 forward (Fig. 141). 



The secreting tubules or true metanephric tubules, are formed 



