116 PRIMITIVE ANIMALS 



a new organ, the allantois (Fig. 24 B al). At the time 

 that the allantois grows out from the foetus the latter 

 is being folded off from the yolk-sac. The embryo is 

 covered over with an investing envelope, the amnion, 

 and between this envelope and the yolk-sac there is 

 a spacious extension of the embryonic coelom to 

 form an extra-embryonic coelomic space. It is into 

 this space that the allantois grows out from the hind 

 gut of the foetus near the region of the umbilicus, 

 pushing with it the coelomic lining of the part of the 

 gut from which it comes. 



Accompanying the allantois, in the coelomic 

 mesoderm, blood-vessels grow out ; the allantois 

 fuses with the thickened trophoblast where the latter 

 has already amalgamated with the uterine tissue, 

 and its blood-vessels vascularize the common mass, 

 and come into the most intimate relation with the 

 maternal blood-vessels. This highly vascular mass 

 of tissue, composed partly of the swollen uterine 

 wall of the mother and partly of the embryonic 

 trophoblast, is vascularized on the one hand by the 

 maternal blood-vessels, on the other by the foetal 

 blood conveyed in the allantoic vessels. The com- 

 posite organ so formed is known as the placenta 

 (Fig. 24 B pi. and pl'.\ and it is by this organ that 

 the real business of the nutrition of the foetus 

 during gestation is carried out. When the allan- 

 tois is fully established, with its blood circulation 



