32 



HUMAN ANATOMY. 



Primitive chorion 



Amnion 



Amniotic sac 



Gut-tube 



primitive chorion is the direct derivative of the original ectodermal layer of the 

 blastodermic vesicle beyond the embryonic region proper, a layer which, on 

 account of this important nutritive function, has been called by Hubrecht the 

 trophoblast. 



As already noted (Fig. 32), the cleft between the parietal and visceral layers 

 of the mesoblast is the primary body-cavity or coelom ; with the separation of these 

 layers following the dorsal and the ventral folding associated respecti\'ely with the 

 formation of the amniotic sac and the gut-tube, the intramesoblastic space becomes 

 greatly expanded and extends between the amnion and primitive chorion. This 

 large space is appropriated only to a limited extent by the future definite body- 

 cavity, and hence is divisible into an embryonic and an extra-embryonic portion, or 

 exocoelom (Fig. 38), which are temporarily continuous. 



The Vitelline Sac. — While the somatopleura is engaged in producing the 

 protecting amniotic sac, the splanchnopleura, composed of the entoblast and the 

 adherent visceral layer of mesoblast, becomes approximated along the ventral sur- 

 face of the embryo to define the primitive gut-tube by enclosing a part of the 

 blastodermic vesicle ; the remaining, and far larger, portion of the latter cavity 

 constitutes the vitelline sac, and corresponds to the yolk-sac of the lower forms. 



The constriction and separation of the gut-tube from the vitelline sac is accom- 

 plished earliest at the 

 ^^^- •59- cephalic and caudal ends 



of the future alimentary 

 canal, the intervening por- 

 tion remaining for a time 

 in widely open communi- 

 cation with the yolk-sac. 

 During the rapid diminu- 

 tion of the latter the com- 

 munication becomes re- 

 duced to a narrow channel, 

 the vitelline duct, which 

 persists as a slender stalk 

 terminating at its distal end 

 in the remains of the yolk- 

 sac. 



In animals other than 

 mammal? in which a pla- 

 centa is developed, the 

 yolk-sac is the chief nutri- 

 tive organ of the embryo ; 

 the mesoblastic tissue of 

 the vesicle becomes vascu- 

 larized by the development 

 of the blood-vessels consti- 

 tuting the vitelline circulation, of which the vitelline or omphalomesenteric arteries 

 and veins form the main trunks. The contents of the yolk-sac as such do not 

 directly minister to the nutrition of the embryo, but only as materials absorbed by 

 the vitelline blood-vessels. In man and other high mammals the nutritive function of 

 the yolk is at best insignificant, the vitelline sac of these animals representing the 

 more important organ of their humbler ancestors. In the lowest members of the 

 mammalian group, the monotremata, in which the large ova are comparatively rich 

 in deutoplasm, the vitelline circulation is of great importance for nutrition, since it 

 constitutes the sole means for this function until the immature animals are hatched and 

 supplied with milk by the mother. In the kangaroo and opossum the yolk-sac at 

 one point forms a disk-like organ, which, from the fact that it becomes provided with 

 vascular villi that lie in contact with the uterine mucous membrane, is termed the 

 vitelline placenta. 



The AUantois and the Chorion. — Coincidendy with the formation of the 

 amnion, another foetal appendage, the allantois, makes its appearance as an out- 



Coelom 



Allantois 



\'itelline duct 



Diagram showing completed amnion and serosa, beginning allantois and 



vitelline duct. 



