THE ALLANTOIS 



349 



The cavity between the false amnion and the true amnion 

 is continuous, with the body cavity at the umbilicus. 



Allantois. The allantois grows out from the back part of 

 the intestinal canal into the celom or the body cavity. (Figs. 



FIG. 106. This and the two following wood-cuts are diagrammatic 

 views of sections, through the developing ovum, showing the forma- 

 tion of the membranes of the chick. (Yeo after Foster and Balfour.) 



A, B, C, D, E, and F, are vertical sections in the long axis of the embryo at 

 different periods, showing the stages of development of the amnion and of the 

 yolk-sac; /, II, III, and IV, are transverse sections at about the same stages of 

 development; i, ii, and in, give only the posterior part of the longitudinal sec- 

 tion to show three stages in the formation of the allantois; e, embryo; y, yolk; 

 pp, pleuroperitoneal fissure; vt, vitelline membrane; af, amniotic fold; al, allan- 

 tois. 



104, 105.) It is of splanchnopleuric origin. It soon be- 

 comes a membranous sac, the walls of which are very vascu- 

 lar. It fills the space between the two amniotic folds and 

 joins the false amnion. Its vessels thus reach the chorion, 

 which is already establishing vascular connections with the 



