IV. VERTEBRATA. 489 



The metanephros has the farther peculiarity of appearing late. The mcso- 

 nephros functions during fcetal life and degenerates accordingly as its work is 

 taken by the metanephros. At last only a part persists, and that in the male, 

 as a few tubules which have united with the testis and now serve as sperm 

 conduits (epididymis) ; farther the Wolffian duct, which forms the vas deferens, 

 carrying the sperm; and a rudimentary structure, the paradidymis, remnants oi 

 nephridial tubules which do not connect with the testis. In the female the 

 whole Wolffian body disappears with the exception of epoophoron and paro- 

 ophoron, corresponding to the epi- and paradidymis, andremainsof the Wolffian 

 duct (Gaertner's canal). 



In close connection with the kidneys, usually in contact with them, are the 

 problematic organs, the siiprarenals. They consist of a cortical portion, derived 

 from the peritoneal epithelium, and a central portion, developmentally connected 

 with the sympathetic system. In the sharks the medullary portion is widely 

 separated from the other. 



The ducts of the urogenital system open behind the anus in most 

 fishes on a urogenital papilla; in the elasmobranchs, amphibians, 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 Amphibia. Urinary and sexual ducts then 

 either open into the urogenital sinus, the lowest part of the bladder leading 

 to the cloaca (turtles, monotremes), or this part receives only the genital 

 ducts, while the ureters enter the base of the bladder. The urogenital 

 sinus remains in connection with the cloaca in the turtles and monotremes; 

 in the other mammals a cloaca occurs only in embryonic life. Later, by 

 formation of the perineum, the cloaca is divided into a hinder digestive and 

 an anterior urogenital canal. 



Asexual and parthenogenetic reproduction are unknown in the vertebrates. 

 The impregnation of the eggs in the lower groups is usually external and occurs 

 during oviposition; in the higher internal copulation is effected by apposition 

 of the genital orifices or by the development of an intromittent organ, the penis. 

 The fertilized egg may undergo a part or the whole of its development in special- 

 ized parts of the oviduct (uterus). Accordingly viviparous and oviparous forms 

 are distinguished, and between these extremes those that are ovoviviparous 

 {cf. p. 151). Most elasmobranchs are viviparous, but many are oviparous. In 

 the teleosts oviparous forms predominate, but there are viviparous exceptions. 

 So, too, among the reptiles and Amphibia there are some viviparous species 

 among the egg-laying majority. The birds and mammals are most constant, 

 the first being exclusively ovoviviparous, while all the mammals with the excep- 

 tion of the ovoviviparous monotremes bring forth living young. 



Three embryonal appendages may occur in the development, the yolk 

 sac, the amnion, and the allantois. The yolk sac is small in those verte- 

 brates which have some yolk, but not enough to cause meroblastic seg- 

 mentation (Amphibia), yet it is everwhere present and is best developed 

 in those groups (fishes, fig. 540; reptiles and birds (fig. 541, do) with 



