notochord 



dermatome 

 myotome 

 nephrocoel 

 pronephric crest 



somatopleure 



opening of nephric duct into cloaca 



B 



Figure 10-21. The origin of the pronephric tubule in Potypterus from the outer layer of the interme- 

 diate mesoderm, A, and the opening of the nephric duct into the cloaca, B. (A, after van den Broek, 

 1 938; B, after Goodrich, 1 930) 



the anterior five rudiments have funnels and their crests 

 join as the precursors of the nephric duct. Several more pos- 

 terior funnels are formed but abort, and the crests of these 

 segments are added to the nephric duct. The nephric duct 

 continues to be extended posteriorly by crest contributions, 

 separated from the intermediate plate, until it reaches the 

 cloaca (Figure 10-21). The most posterior contributions ap- 

 pear to represent the entire intermediate plate, which then 

 hollows out. 



As segmental coelomic spaces develop, only the second 

 and fifth pronephric funnels remain, and their nephrocoels 

 open laterally into the coelom through peritoneal funnels. 

 Each of these remaining tubules develops a renal corpuscle. 

 With further growth the two units of a side fuse to form a 

 single element having a single glomerulus and a very con- 

 voluted tubule. As this process of fusion goes on, the con- 

 nection between the nephrocoel (now Bowman's capsule) 

 and the coelom is lost. This single pronephric tubule lies in 

 the fifth somite. 



The mesonephros develops from segmental clumps of 

 nephrogenic tissue, beginning about four somites behind 

 the single pronephric tubule and extending through the 

 39th somite. The primary tubules differentiate and enter 

 the nephric duct. In the anterior somites of the mesonephros 

 there are usually two glomeruli and as many tubules. In the 

 caudal half there are more glomeruli (as many as five) with 

 the same number of tubules. In the 20th to 36th somites 

 there are peritoneal funnels for each of the tubules. 



The pronephros has disappeared in a 90-mm larva and 

 peritoneal funnels are present as late as 150-mm but are 

 lacking in the adult. The number of tubules has greatly in- 

 creased and many use a single collecting duct. 



Acipenser The kidney of this fish is long and tapered, thin 

 anteriorly and widened posteriorly where the two organs are 

 fused (Figure 10-45). The anterior end is continued for- 

 ward into the region of the pronephros, under the posterior 



end of the chondrocranium. The kidney is possibly a holo- 

 nephros; that of Polyodon is said to be a holonephros. 



The nephric duct increases in diameter posteriorly and is 

 suddenly expanded laterally as a bladder which extends 

 most of the length of the fused portion. Posteriorly the two 

 ducts unite to form a midline sinus. In both sexes a large 

 ciliated funnel joins the nephric duct near the anterior end 

 of the expanded bladder portion. This funnel serves for pas- 

 sage of the eggs in the female but is not a genital duct in 

 the male. 



In Polyodon this funnel enters the posterior part of the ex- 

 panded bladder portion of the nephric duct, just anterior to 

 the midline urinary sinus. In young specimens it ends blindly. 

 In one female the duct had failed to form on one side. 



The testis of the male is connected with the kidney tu- 

 bules by a large number of vasa efferentia. These enter a mar- 

 ginal canal from which a number of ducts pass into the 

 substance of the kidney and enter the capsules of some of 

 the tubules. More than the anterior half of the kidney 

 length is involved in the cross connection between testis and 

 nephric duct; about one tubule in three is involved in 

 sperm transport and these retain their glomeruli. 



EMBRYOLOGiCAL DEVELOPMENT The pronephros arises in 

 postotic segments 4 to 8 or 3 to 10 (Figure 10-22). As al- 

 ready observed in the salmon, the outer layer of the inter- 

 mediate plate, or the somatic mesodermal layer, evaginates 

 upward and laterally to form a crest. The crests of the 

 several pronephric segments join to form a ridge or strand 

 which is continued posteriorly by comparable contributions 

 fi-om the intermediate plate of the more posterior segments, 

 all the way to the cloaca. The ridge separates from the 

 lateral plate and hollows out to form the nephric duct. 



The first five to seven of the pronephric segments develop 

 a nephrocoel in the intermediate plate as well as a funnel. 

 The first several nephrocoels open into the coelom. The 

 most anterior funnel becomes vestigial very early. Later the 



THE EXCRETORY SYSTEM 



307 



