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—FETAL 
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 aira 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 
