THE VITELLINE SAC. 33 



growth from the caudal segment of the primary gut-tract. Although modified in 

 man and certain mammals to such an extent that its typical form and relations are 

 obscured, the allantois, when developed in a characteristic manner, as in the chick, 

 assumes the appearance of a free vesicle connected with the embryo near its caudal 

 pole by means of a narrow pedicle, the allantoic stalk. Since the allantois is an 

 evagination from the primitive gut, its walls are formed by direct continuations of 

 the primary layers enclosing the digestive canal, — namely, a lining of entoblastic 

 cells, reinforced externally by a layer of visceral mesoblast. 



Beginning as a wide bay on the ventral wall of the hind-gut, the allantois elon- 

 gates and appears as a pyriform sac projecting from the embryo behind the attach- 

 ment of the still large vitelline stalk (Fig. 39). It rapidly grows into the exocoelom, 

 and in mammals expands in all directions until it comes into contact with the inner 

 surface of the primitive chorion, with which it fuses to constitute the true chorion. 

 The latter, sometimes spoken of as the allantoic chorion in contrast ta the amniotic 

 or primitive chorion, now becomes the most important envelope of the mammalian 

 embryo on account of the role that it is destined to play in establishing the respira- 



FiG. 40. 



-Primitive chorion 

 -Amnion 



Amniotic sac 



Allantois 



^Vitelline sac 

 Diagram showing villous condition of serosa, expanding allantois, and diminishing vitelline sac. 



tory and nutritive organ of the foetus, the placenta. After the fusion of the allantois 

 with the primitive chorion to form the chorion, the villous projections upon the 

 external surface of the latter become more highly developed, consisting of a core of 

 mesoblastic tissue covered externally by the ectoblast. 



The primary purpose of the allantois, as a receptacle for the ef?ete materials ex- 

 creted by the Wolffian body of the early foetus, is soon overshadowed by its function 

 as a respiratory organ ; this occurs with the appearance of the rich vascular supply 

 within the chorion following the invasion of its mesoblastic tissue by the blood-vessels 

 constituting the allantoic circulation. The latter includes the two allantoic arteries, 

 which are extensions from the aortic stem of the embryo and convey venous blood, 

 and the two allantoic veins, which return the oxygenated blood to the embryo and 

 become tributary to the great venous segment of the primitive heart. The vascu- 

 larization of the chorion extends to the highly developed villi occupying its outer 

 surface in many mammalian forms, especially man. 



The vascular villi of the chorion, bearing the terminal loops of the blood-vessels 

 conveying the foetal blood, are important structures on account of their intimate 

 relations vA\\\ the uterine mucous membrane (Fig. 41), in conjunction with which 



3 



