midline glomerulus 



^coelom j^^^gi (4,h somite) 



nephric duct 



funnels opening into 

 nephrocoels 



mesonephric tubule 



coelom 



nephrocoel 

 (10th somite) 



Figure 10-22. Development of the kidney in Acipenser. A, entire pronephric system with meso- 

 nephric beginnings in 1 3-mm larva; B, pronephric tubules opening into general coelom or coelomic 

 pockets at 26-hr stage; C, D, and E cross sections showing relationship between pronephric elements 

 and coelom. (A, after Kindahl, 1838; B-E, after Maschkowzeff, 1926) 



remaining funnels, with the exception of the most anterior 

 one, lose their coelomic connection. The several pronephric 

 tubules become associated with glomeruli lying just ventro- 

 lateral to the dorsal aorta. The more anterior segments of 

 the pronephros now appear to undergo some posterior dis- 

 placement. The glomeruli unite to form a single large median 

 glomus lying immediately below the dorsal aorta, and the 

 nephrocoels fuse into a single cavity on either side of this. 

 At this time the pronephros undergoes degeneration, but 

 possibly passes through a stage with a single pair of pro- 

 nephric units formed by fusion of the several earlier ones. 

 The mesonephros is developed from segmental blastemal 

 knots. These form tubules which join the nephric duct. Each 

 tubule has a renal corpuscle and a peritoneal funnel. Multi- 



plication of tubules occurs through secondary and tertiary 

 (and higher) generations which use the primary tubule as a 

 collecting duct. The peritoneal funnels are lost in the adult, 

 but the genital duct, which is similar in both male and fe- 

 male, may functionally replace them. 



It has been suggested that the genital duct (of the female) 

 is developed from the rudimentary peritoneal funnels of the 

 secondary tubules or even by division of the nephric duct. 

 It appears to be comparable to that of Lepisosletis and devel- 

 ops in the same way. 



Amia and Lephosfeus The adult kidney of Amta is an 

 opisthonephros. These organs are fused posteriorly into a 

 single mass which extends behind the anal opening. Ante- 



308 



THE UROGENITAL SYSTEM 



