628 MAMMALIAN EMBRYOLOGY 



by the broad yolk-stalk just as with the chick. Its wall is separated from 

 the chorion by the extra-embryonic coelom also called the exocoelom. 

 (Fig. 359, E.) 



The amnion and chorion are formed from somatopleure, while the 

 yolk-sac is formed from splanchnopleure. 



The blood vessels and the sinus terminalis arise in the yolk-sac of 

 '.he rabbit just as they did in the chick. 



In the higher primates, including man, the yolk-sac never fills the 

 entire blastodermic vesicle and is very slow to grow. In fact, during 

 the first month it has a diameter about the length of the embryo, and 

 after increasing this diameter to a little over a centimeter, it decreases in 

 size. The yolk-stalk is formed, however, and elongates considerably to 

 enter the proximal end of the umbilical cord. 



The amniotic membrane now' expands and pushes against the exo- 

 coelom until that is eliminated and the yolk-sac disappears in the pla- 

 cental region. The yolk-stalk itself becomes a solid cord during the 

 second month. However, the proximal end sometimes remains open. 

 In such a case it appears as a diverticulum from the intestine, and is 

 called Meckel's diverticulum. 



THE ALLANTOIS 



This structure also varies in size to a considerable extent, from 

 filling the entire exocoelom as in the lower primates such as the Lemurs, 

 to occupying but a small portion of the umbilical cord as in man and the 

 higher primates (Fig. 359, G). 



The early development of the allantois in the mammals is quite sim- 

 ilar to that in the chick, but its later development is varied, the variation 

 being ascribed to the changed conditions brought about by the formation 

 of placental structures. 



The later history of the allantois is limited to the placental struc- 

 tures only. In the rabbit the allantois extends into the exocoelom and 

 comes in direct contact with the chorion in the region where chorion 

 and uterus unite. It thus lies in the direct pathway of connection be- 

 tween mother and offspring. Blood vessels now develop in the allantoic 

 mesoderm to form the umbilical arteries and the umbilical veins, and it 

 is through these allantoic blood vessels that the embryonic circulation 

 * ; .s related to the placental circulation. 



In man the development is quite different ; for, here there is nothing 

 .vhich interrupts the connection of chorion with the maternal tissues. 

 The way in which the body-stalk develops in man has been described 

 already. This is often said to be equivalent to a modified allantoic stalk. 

 There is, therefore, in man, no true allantois as a free vesicle. Only a 

 small tubular outgrowth from the entodermal lining of the yolk-sac can 

 be seen, and this outgrowth, in turn, is not distinguishaable from the 

 hind-gut. It extends in to the body-stalk. As the embryo grows, and the 



