DEVELOPMENT OF THE FETAL MEMBRANES AND THE PLACENTA 55 



In reptiles, birds, and many mammals the allantois becomes expanded into a 

 vesicle which projects into the extra-embryonic celom. If its further development 

 be traced in the bird, it is seen to project to the right side of the embryo, and, 

 gradually expanding, it spreads over its dorsal surface as a flattened sac between 

 the amnion and the serosa, and extending in all directions, ultimately surrounds 

 the yolk. Its outer wall becomes applied to and fuses with the serosa, which lies 

 immediately inside the shell membrane. Blood is carried to the allantoic sac by 



Amniotic cavity 



Amniotic cavity 

 Yolk-sac 



Chorion 



FIQ. -4. Diagram showing earliest observed stage 

 of human ovum. 



Body-stalk 

 Allantoic 



Yolk-sac 



Chorion 



FIG. 25. Diagram illustrating early formation of 

 allantois and differentiation of body-stalk. 



the two allantoic or umbilical arteries, which are continuous with the primitive 

 aortas, and after circulating through the allantoic capillaries, is returned to the 

 primitive heart by the two umbilical veins. In this way the allantoic circulation, 

 which is of the utmost importance in connection with the respiration and nutrition 

 of the chick, is established. Oxygen is taken from, and carbonic acid is given up 

 to the atmosphere through the egg-shell, while nutritive materials are at the same 

 time absorbed by the blood from the yolk. 



Amniotic cavity 

 Embryo 



Body-stalk 



Placental 

 villi 



Placental 



villi 



Chorion 



Body-Stalk 



Allantois 

 Yolk-sac 



Heart 



Heart 



FIG. 26 Diagram showing later stage of allan- 

 toic development with commencing constriction 

 of the yolk-sac. 



Fore-gut 

 Embryo 

 Amniotic cavity 



Fio. 27. Diagram showing the expansion of amnion 

 and delimitation of the umbilicus. 



In man and other primates the nature of the allantois is entirely different from 

 that just described. Here it exists merely as a narrow, tubular diverticulum of the 

 hind-gut, and never assumes the form of a vesicle outside the embryo. With the 

 formation of the amnion the embryo is, in most animals, entirely separated from 

 the chorion, and is only again united to it when the allantoic mesoderm spreads 

 over and becomes applied to its inner surface. The human embryo, on the other 

 hand, as was pointed out by His, is never wholly separated from the chorion, its 



