limit of neural crest mesoderm (dashed line) 

 otic capsule 



pharyngeal pouches 



somatopleure enclosing coelom 



pronephric nephrostomes 



nephric duct 



nephric blastema 



Figure 10-13. Mesodermal components in a larval salamander. (After Witschi, 1956) 



ric duct, open separately into the cloaca, or join together 

 as a single ureter to enter the cloaca. The development of 

 this posterior part of the opisthonephros in no way suggests 

 the metanephros. The pronephros degenerates with the de- 

 velopment of the posterior part of the opisthonephros. 



In the posterior part of the kidney of the apodan amphib- 

 ian, the primary tubules connect with outgrowths from the 

 nephric duct. These outgrowths resemble in their origin the 

 single diverticulum of the metanephric system of the amni- 

 ote. As a note here it should be pointed out that Brauer's 

 (1902) work with the apodan Hypogeophis is a classic study 

 of the development of an amphibian kidney, and from this 

 a pattern for the differentiation of the mesoderm has been 

 derived (Figure 10-14). This pattern suggests that the fun- 

 nels of the pronephric tubules are not homologous to the 

 peritoneal funnels of the mesonephric tubules and that the 

 nephrocoel in the more advanced tubules has been reduced 

 to the Bowman's capsule. 



The Mullerian duct first appears as a thickening of the 

 epithelium lateral to the nephric duct; this thickening forms 

 a groove which closes over and gradually extends posteri- 

 orly, in both sexes, to open into the cloaca in the adult stage. 



Frog The opisthonephros of the frog is a compact elongate 

 ovate body. It is drained by the nephric duct lying along its 

 outer margin. This duct also serves the testis of the male as 

 a vas deferens. Relationships with the testis are variable 



among anurans (Figure 10-15). In a few species a ureter is 

 developed. Peculiar to the anuran is the ampulla of the 

 nephric duct, a swollen glandular section just before the 

 tube enters the cloaca. Peritoneal funnels are present in the 

 adult but are unique in that they open into the venous 

 sinuses (Figure 10-17). 



neural crest 



sclerotome 

 sclerocoel 



myotome 

 dermatome 

 myocoel 

 nephric tubule 

 nephric duct 



nephric funnel 



coelom 

 peritoneal funnel 



nephrocoel 

 (Bowmen's capsule) 

 Figure 10-14. Mesodermol components as suggested in the develop- 

 ment of Hypogeophis, an apodan amphibian. (After van den Broek, 

 1938) 



THE EXCRETORY SYSTEM • 301 



