348 COMPARATIVE MOGPHOLOGY OF VERTEBRATES. 



close of sexual life is not a case of hermaphroditism.) Among the 

 mammals the cases are extremely rare, but cases, apparently well 

 authenticated, have been reported in the goat, pig and man. 



NUTRITION AND RESPIRATION OF THE EMBRYO FOETAL 



ENVELOPES. 



In all vertebrates except the mammals there is enough nourish- 

 ment stored in the egg to carry the young through its development 

 up to the point where it hatches and shifts for itself. In the cyclo 

 stomes, dipnoi and amphibia this nourishment (food-yolk or deuto- 

 plasm) is soon enclosed in the body wall. In ganoids and teleosts, 

 where it is relatively larger in amount, it forms for a time a projecting 

 mass enclosed in a yolk sac, and this condition reaches its extreme in 

 the elasmobranchs and sauropsida. The yolk sac, in the fishes, is an 

 extension of the intestine and the body wall and is richly supplied by 

 vitelline arteries and veins which are derivatives of the omphalo- 

 mesenteric vessels (p. 276). In the sauropsida, owing to the develop- 

 ment of the amnion and the consequent separation of the non- 

 embryonic somatopleure from the yolk, the yolk sac is composed of 

 the splanchnopleure alone, but it has homologous blood-vessels. In 

 the mammals (monotremes excepted) the yolk is greatly reduced and 

 the yolk sac (here often called the umbilical vesicle) is vestigial in 

 character. 



The vitelline vessels take the yolk and carry it into the body where 

 it is utilized in building the embryo, all of it being eventually metabo- 

 lized and used by the cells. The rich supply of capillary vessels in the 

 sac also forms an efficient respiratory apparatus. In the viviparous 

 sharks villi are developed on the oviducal lining and these afford a 

 means of exchange of gases with the embryo, and for getting rid of the 

 nitrogenous waste. It is a question how far there is a transfer of food 

 by the same means. In some species of Mustelus and Carcharias 

 the villi fit into depressions in the yolk sac, thus forming an analogue 

 to the placenta of the mammals a vitelline placenta though formed 

 in a greatly different manner. 



The viviparous teleosts have saccular ovaries and the development 

 of the egg takes place in the cavity, the walls of which at the breeding 

 season become villous. In the viviparous Salamandra atra only one 

 egg develops and this leaves the mother in the adult shape. The 

 other eggs degenerate and are used as food by the one. There is also 



