COMPARATIVE ANATOMY OF THE UROGENITAL SYSTEM 



275 



described them. These develop in the mesomere immediately posterior to the pronephroi, even 

 overlapping the posterior ends of the pronephroi. They extend over a number of segments, 

 and are much larger than the pronephroi. As they develop, the pronephroi generally degen- 

 erate. The mesonephros consists of many tubules, a number to each segment ; these, like the 

 pronephric tubules, open at one end into the coelom by nephrostomes and at the other end 

 terminate in the pronephric duct (Fig. 6o#). Although the pronephroi vanish, the pronephric 

 ducts persist; they grow posteriorly alongside the region where the mesonephroi are developing, 

 and become the ducts of the latter. They are then known as the mesonephric or Wolffian duds. 

 The mesonephric duets grow posteriorly until they reach the caudal end of the intestine. 

 To this they become connected, pouring their contents into the terminal chamber of the 

 intestine, which consequently receives the name of cloaca (Latin, "a sewer"). The structure 



neural tube 



epimere 



cpimere 

 notochord 



dorsal aorU 



mesonephric 

 duct 



nephrostome 



mesonephric 

 tubule 



Bowman's capsule 

 internal 

 glomerulus 



pronephric 

 tubule 



pronephric 

 duct 



nephrostome 



MESONEPHROS 



intestine 



FIG. 61. Diagram to show the structure of the pronephros and mesonephros. Pronephros on the 

 right, mesonephros on the left. The chief difference is in the relation of the glomerulus; in the pro- 

 nephros it projects into the coelom; in the mesonephros it projects into the tubule, which forms a 

 Bowman's capsule about it. (From Wiedersheim's Comparative Anatomy of Vertebrates, courtesy of 

 the Macmillan Company.) 



of the mesonephros differs from that of the pronephros. The tubules are more numerous, 

 longer, and more convoluted. The glomeruli are always of the internal kind (Fig. 61). Each 

 glomerulus projects into the mesonephros, carrying the wall of the tubule before it to form a 

 flask-shaped capsule in which the glomerulus rests. The capsule is named Bowman's capsule, 

 and this, together with the glomerulus, forms a rounded body known as the Malpighian body 

 or renal corpuscle. The mesonephric tubules generally open into the coelom by nephrostomes 

 which are situated near the renal corpuscles. 



The mesonephroi are the kidneys of the adults of fishes and Amphibia, and are also the 

 chief functional kidneys of cyclostomes, although the latter possess in addition persistent 

 pronephroi. The mesonephric or Wolflian ducts are the excretory ducts in these vertebrates, 

 opening into the cloaca. The mesonephroi appear in the embryos of reptiles, birds, and 

 mammals, and are in most cases functional in the embryos, often becoming of large size. 

 They generally, however, lack nephrostomes. They degenerate before hatching or birth, or 

 very shortly after. 



