THE LATER DEVELOPMENT OF THE FROG 



195 



directly into the cavity of the pronephric duct, which is con- 

 tinuous posteriorly with this portion of the pronephric 

 rudiment. 



There are now established the primary elements of the 

 pronephros. The three short canals with their cellular walls 

 are the rudiments of the three pronephric tubules, and their 

 openings into the coelom are the nephrostomes. The tubules 

 and also the proximal part 

 of the pronephric duct, 

 now elongate rapidly, and 

 as a consequence become 

 thrown into complicated 

 loops and folds (Figs. 71, 

 72) forming a conspicuous 

 mass whose position is 

 marked externally by a 

 slight elevation. The pro- 

 nephric duct gradually ac- 

 quires a lumen throughout 

 its extent; at about 4.5 

 mm. the duct effects a 

 connection with the wall 

 of the cloaca, and its 

 cavity then opens into the 



Cbacal Chamber (Fig. 54). FlG . n ._ To tal views of the pronephros 



The nephrostomes become of the frog (R. sylvatica). After Field. A. 



... ... , . , Right pronephros of an embryo of about 3.5 



lined With large Cilia Which mm. The crosses mark the location of the 

 nrnrhipp f\ onrrpnt out of nephrostomes. B. Right pronephros of a 



* larva of about 6 mm. First tubule dotted; 



the COelom, passing by Way second white; third obliquely ruled; pro- 

 ,. , , i j nephric (segmental) duct shaded with lines. 



of the pronephric duct to 

 the cloaca. 



Meanwhile the pronephros acquires a rich vascular supply, 

 both arterial and venous. It will be remembered that the 

 posterior cardinal veins are passing along the pronephric ducts, 

 and in the region of the pronephros itself these veins become 

 greatly enlarged. As the pronephric tubules elongate, they 

 push up into the posterior cardinal sinus, which is ultimately 



