552 CHORDATA. 



In the males (excepting many fishes) the testes become con- 

 nected with the anterior end of the Wolffian body (fig. 581, A), 

 so that the urinary tubules of the latter come to be seminal ducts, 

 while the hinder portion remains excretory, this condition being 

 permanent in the Amphibia. In the amniotes the anterior meso- 

 nephros retains its connexion with the testes, forming the vasa 

 efferentia, while the Wolffian duct forms the vas deferens, a por- 

 tion of it greatly coiled being the epididymis. The remainder 

 of the Wolffian body degenerates, a portion only persisting as the 

 paradidymis. 



In the females (fig. 581, B) the mesonephros is smaller in front, 

 as in the males, but the connexion of this with the ovary does not 

 exist, so here the Wolffian duct is solely excretory, and not, as in 

 the males, excretory and seminal duct. In the female amniotes 

 the Wolffian body almost entirely disappears, for in both sexes of 

 the reptiles, birds, and mammals the metanephros or kidney proper 

 is a new formation, growing forwards from the posterior end of 

 the Wolffian duct. In the females of elasmobranchs, Amphibia, 

 and Amniotes the Miillerian duct serves as an oviduct, its anterior 

 end opening by the ostium tubae into the abdominal cavity and 

 receiving the eggs as they escape from the ovary. In the male the 

 Miillerian duct disappears early. 



The union of sexual and excretory organs to a urogenital system arises 

 from the same relations as in the annelids ; both organs arise from the 

 coelomic epithelium and have temporary or permanent connexion with the 

 body cavity. This has already been described for the gonads. The 

 urinary tubules of both pro- and mesonephros are derivatives of the coelomic 

 epithelium and possess an arrangement recalling that of the annelids in 

 a striking manner. As is shown (fig. 70) in the scheme of the embryo 

 selachian, the nephridial system consists of numerous canals, segmeu tally 

 arranged, connected by funnels (nephrostomes) with the body cavity; 

 and differs from the segmental organs of the annelids in that they do not 

 open singly to the exterior, but by a common duct. They also differ in 

 their further development by increasing greatly in number and forming 

 a compact organ, and, finally, by the formation in a certain part of a 

 network of blood-vess'els, the glomerulus, which pushes into the lumen of 

 the tube. 



The ducts of the urogenital system open behind the anus in 

 most fishes on a urogenital papilla; in the elasmobranchs, amphib- 

 ians, birds, and most reptiles dorsally into the hinder part of 

 the digestive tract, which thus becomes a cloaca. In turtles and 

 mammals the urogenital canal opens into the urinary bladder, a 

 ventral diverticulum of the rectum which first appears in the 



