THE EARLY DEVELOPMENT OF THE MAMMAL 435 



exoccelom opposite the posterior end (primitive streak) of the 

 embryo (Fig. 176, -A). At its base the tail-fold of the amnion 

 is just appearing, and there is already a slight indication of the 

 evagination of the endoderm into it. By the ninth day the 

 tail of the embryo has begun to grow out, and the allantoic 

 rudiment is forced into a ventral position; the amnion therefore 

 appears to arise posteriorly to the allantois, beneath the base 

 of the tail. By this day the allantois has enlarged and its 

 extremity has dilated forming a vesicle which extends freely 

 into the exoccelom, while the narrower allantoic stalk is 

 attached beneath the embryo, just back of the attachment of 

 the yolk-sac, from the cavity of which the allantoic cavity is 

 now clearly marked off. 



As the allantois grows out it comes immediately into relation 

 with the inner surface of the chorion, in the region where the 

 chorionic ectoderm has become trophodermic (Fig. 176, 5), and 

 since the trophoderm is the beginning of the placental struc- 

 ture, the allantoic stalk thus becomes the direct pathway 

 between the embryo and the placental region. 



During the tenth to twelfth days the allantois expands 

 rapidly beneath the chorionic trophoderm, its mesodermal wall 

 thickens and in it a rich vascular network is developed (Fig. 

 176, C). Blood vessels have been present in the allantoic meso- 

 derm from a very early period, and by the tenth day there are 

 present a pair of umbilical arteries, and a pair of umbilical veins, 

 having relations similar to those already described in the human 

 embryo. It is through the allantoic blood vessels, therefore, 

 that the embryonic circulation is related with the placental, 

 and in the rabbit this appears to be the only important function 

 of the whole allantoic structure. 



In man the history of the allantois is a very different story. 

 Here, as in all Primates, the primitive connection of the hinder 

 end of the embryo with the chorion is never interrupted, and 

 this connection, known as the body stalk (see above), composed 

 of mesoderm, may be regarded as the modified equivalent of 

 the allantoic stalk of such a form as the rabbit. Into this body 

 stalk there early extends a small tubular outgrowth from the 



