THE EMBRYONIC MEMBRANES 249 



the disappearance of the shell. It is necessary to mention this 

 last proviso because in some forms the egg is not laid ; it 

 undergoes development in the oviduct but does not lose the 

 shell. This condition, which occurs in the viper, is called 

 ovo-viviparous. 



In the Placental mammals, the allantois relieves the yolk-sac 

 in the formation of the placenta, and the higher the order of 



Fig. 116. — Diagram of the relations of the embryonic membranes in the 

 human embryo. (From Jenkinson, after Graf Spee.) 



The embryo, developed in the floor of its amniotic cavity (amc) is 

 attached to the trophoblast by the mesodermal body-stalk (bst), into which 

 the allantois is beginning to grow ; bv, blood-vessels round the wall of the 

 yolk-sac (ys) B, transverse section through the body-stalk in the plane 

 indicated. 



mammals the earlier does this happen. Indeed, in the highest 

 of all, the Primates (including man), the mesoderm of the 

 allantoic stalk appears from the beginning (the " body-stalk "), 

 and the endodermal allantois grows into it later. In these 

 animals the allantoic blood-vessels (the umbilical arteries and 

 veins) are ready at a very early stage to transport to and from 

 the embryo, which increases the efficiency of the placenta. 

 The blood-vessels of the allantois are usually covered by 



