380 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



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 utiHzed in building the embryo, all of it being eventually met- 

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

 the sac also forms an efficient respiratory apparatus. In the vivi- 

 parous 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 develop- 

 ment 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 a modification of the lining of the oviduct in this species which 

 allows some blood to escape and this gives additional nourishment. 



In the amniotes the yolk sac reappears and there are, in addition, 

 two other foetal structures which are peculiarly characteristic, the 

 allantois and the amnion, to which reference has been made in the 

 preceding pages. 



The amnion usually arises as a fold of the somatic wall of the 

 coelom in front and on either side of the embryo (fig. 404), later 

 followed by a similar tail fold. These folds extend upward and then 

 inward, until they finally meet above the embryo, thus enclosing it 

 in an amniotic cavity, filled with an amniotic fluid. These folds 

 fuse in the middle line, the two sides of the fusion breaking through 

 so that above the wall of the amniotic cavity — the true amnion — 

 there is a second cavity, a part of the extraembryonic coelom, directly 

 continuous with the ccelom of the body. This cavity is bounded 

 externally by the dorsal part of the amniotic fold, which is called 

 the serosa, false amnion or chorion. The serosa lies immediately 

 below the vitelline membrane of the egg. 



In a number of mammals the amniotic cavity arises by a splitting of the 

 embryonic area in such a way that the results are practically similar to those 



