pronephric glomerulus 



nephric duct 



LOPHIUS PISCATORIUS 



B 



HIPPOCAMPUS 



Figure 10-19. Tubules of aglomerular kidneys in two teleost fishes. A, iophius, and B, Hippocam- 

 pus. (After Gerard, 1954) 



ones of which produce rudimentary tubules which abort. 

 The functional kidney lies from the 14th somite back to 

 about segment 38; four segments of this are postcloacal. 

 The excretory regions of the kidneys fuse across the midline. 

 Like the higher groups, except the mammals, the blood en- 

 tering the kidney is both arterial and venous. 



Polypterus The opisthonephros of this type is long and 

 thin, and the organs of either side are not joined posteriorly 



(Figure 10-45). The nephric duct is distinct from the geni- 

 tal duct in both the male and female. The nephric ducts 

 are expanded and joined to form a urinary sinus, which 

 opens into the cloaca. 



EMBRYOLOGiCAL DEVELOPMENT The pronephros is formed 

 by seven to nine nephrotomes, each having a nephrocoel 

 (Figure 10-20). The outer layer of the nephrotome evagi- 

 nates to form a pronephric crest and a funnel (Figure 10-21); 



rudimentary 

 pronephric tubules 



rudimentary nephrocoel 



pronephric tubules and nephrocoels 



glomerulus 



nephric duct 



4- somite gap 

 anterior to 

 mesonephros 



Figure 10-20. Four stages in the development of pronephros in Po/ypterus. (After von den Broek, 



B 



1938) 



306 



THE UROGENITAL SYSTEM 



