348 



REPRODUCTION 



branous sac, the walls of which are very vascular. 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 mother. Finally they are distrib- 



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

 of sections, through the developing ovum, showing the formation 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 differ- 

 ent periods, showing the stages of development of the amnion and of the yolk-sac; 

 I, II, III, and IV, are transverse sections at about the same stages of development; 

 i, ii, and iii, give only the posterior part of the longitudinal section to show three 

 stages in the formation of the allantois; e, embryo; y, yolk; pp, pleuroperitoneal 

 fissure; vt, vitelline membrane; a/, amniotic fold; a/, allantois. 



uted only to a certain part (placenta) of the chorion; and as the 

 allantoic vessels anastomose more and more freely with those 

 of the chorion, the umbilical vesicle shrivels, as it is no longer 

 needed. The vessels of the allantois are the two allantoic 



