duct of the male extends anteriorly a short way as well as 

 posteriorly from the epididymis. It is very swollen and con- 

 voluted in the region of the epididymis. 



Posteriorly the nephric duct drains the enlarged and ex- 

 cretory part of the opisthonephros. There are no separate 

 posterior ureters in Neclurus, but in Cryptobranchus a series of 

 these is developed in both males and females. These drain 

 the posterior, excretory part of the kidney directly into the 

 cloaca. The nephric duct is thus separated as a vas deferens. 

 The opisthonephros of the salamander is peculiar in that 

 peritoneal funnels are present in the adult. In the males these 

 regress in the anterior epididymal part of the kidney at the 

 time of reproduction. The kidneys of frog and salamander 

 receive both venous and arterial blood; in this respect they 

 resemble the reptile (Figure 10-12). 



EMBRYOLOGiCAL DEVELOPMENT Pronephric development be- 

 gins with the appearance of cell masses (pronephric crests) 

 bulging from the anterior nephrotomes (intermediate 

 plate of mesoderm). Segmental nephrocoels form in these 

 segments and connect with the coelom (Figure 10-15). 

 These segmental cavities soon become lost in the coelom. 

 Funnels form in postotic segments 4 and 5 (spinal segments 

 1 and 2). The number of pronephric funnels shows some 

 variation; there are four in Crvplobranchiis and the apodan 

 Hvpogeophis has even more. These funnels open into a pro- 

 nephric duct formed by the confluence and hollowing out 

 of the segmental pronephric crests. This duct is continued 

 posteriorly by similar segmental nephrotomic contributions 

 until it reaches the cloaca. 



The pronephric tubules become quite elongated and 

 looped as does the pronephric duct. These tubes come to lie 



in the sinus of the anterior cardinal vein. The two ciliated 

 pronephric funnels of the typical salamander (Figure 10-13) 

 become associated with a large medial glomerulus project- 

 ing on either side of the median septum into the coelom. 



The opisthonephros develops from the segments behind 

 the pronephros but is separated from the pronephric area 

 by a gap. This is explained by the fact that the more pos- 

 terior pronephric segments do not produce funnels. Further- 

 more, the most anterior segments of the mesonephros are 

 only slightly developed and tend to degenerate early. 



The sequence of tubule formation is from anterior to pos- 

 terior, but the most anterior elements are somewhat retarded 

 and tend to abort early. The tubules arise from small 

 masses of blastema which hollow out to form S-shaped tu- 

 bules, each with a Bowman's capsule medially and an attach- 

 ment to the nephric duct laterally. These tubules develop 

 a ciliated peritoneal funnel as an outgrowth from the rudi- 

 mentary capsule. 



The anterior end of the mesonephros never becomes 

 excretory. Its tubules develop secondary and tertiary side 

 branches which end blindly and the renal corpuscles of 

 some tubules abort when connection with the testis is 

 achieved. The efferent ducts of the apodan, however, retain 

 the glomeruli. Some of the mesonephric tubules lie anterior 

 to the efferent ducts and these degenerate in later stages. 



In the posterior part of the kidney the primary tubules 

 are not segmentally arranged and are quite erratic in num- 

 ber and position. These tubules are followed by secondary, 

 tertiary, and higher orders which use the primary ones as 

 collecting ducts. The connections of these primary tubules 

 tend to be displaced posteriorly in some salamanders and 

 thus give rise to the ureters which may open into the neph- 



afferent vein 



renal portal 



proximal convoluted tubule 



Henle's segment 



distal convoluted tubule 

 efferent vein 



dorsal aorta 



postcovo (subcardinal) 



mesovarium 



nephric duct 



nephrostome 

 proximal convoluted tubule 



Figure 10-12. Cross section of kidney of Necturus mocu/osus showing relationships of the tubules 

 to the blood supply. (After Chase, 1923) 



300 • THE UROGENITAL SYSTEM 



