XIV ELEMENTS OF VERTEBRATE EMBRYOLOGY 471 



with the body of the embryo by the slender stalk of the allantois, 

 which becomes much elongated and is enclosed along with the stalk of 

 the yolk-sac in a tubular sheath of somatopleurej continuous on the 

 one hand with the amnion and on the other with the body of the 

 embryo. This somatopleural tube is known as the umbilical cord. At 

 birth this is severed and the placenta together with the lining of the 

 rest of the uterus is shed a little later as the after-birth. 



It has already been pointed out that the general arrangements in the 

 Rabbit blastocyst correspond closely with those in an early stage in 

 development of the bird but that an important difference exists inasmuch 

 as the yolk-sac is empty of yolk. As the body of the embryo increases 

 in size the upper wall of the yolk-sac becomes pressed down against the 

 lower and about the fourteenth day the latter begins to disintegrate and 

 eventually disappears completely. When this process is accomplished 

 a large proportion of the blastocyst surface consists of endoderm, 

 exposed by the disappearance of the lower wall of the yolk-sac, and this 

 probably plays a part in the nourishment of the embryo, absorbing 

 food-material from the fluid within the uterus and passing it on to the 

 body of the embryo by the rich network of blood-vessels in its meso- 

 dermal coat. These blood-vessels — constituting the vascular area — are 

 confined to the upper wall of the yolk-sac, the mesoderm coating of the 

 yolk-sac never extending beyond its margin. 



Having now sketched the main peculiarities in the development of 

 a Bird and a Mammal it will be of interest to take a more general view 

 of these peculiarities in the Amniota as a whole. The mode of develop- 

 ment seen in the Fowl holds not only for other birds but for the great 

 majority of Reptiles. But it is of much interest to notice that even 

 within the group Reptilia there are occasional genera which have become 

 viviparous, the egg undergoing its development within a uterus. This 

 holds for various snakes (e.g. the Adder — Vipera) and Lizards (e.g. 

 Chalcides). Wherever this happens, and where in consequence vascular 

 surfaces of embryo and mother come into close apposition, we might 

 expect that the apposed surfaces would tend to come into still more 

 intimate relations and develop something of the nature of a placenta 

 to facilitate respiratory exchange between the embryonic and maternal 

 blood-streams. This actually happens — to an extent differing in the 

 different Reptiles in question, e.g. in Chalcides tridactylus, a common 

 Lizard of Southern Europe, a definite allantoic placenta is formed while 

 placental formation takes place in addition over the surface of the yolk- 

 sac, the embryonic surface in each case closely interlocking with the 

 lining of the uterus. 



