AMNION AND ALLANTOIS. 



601 



Fig. 208. 



FECUNDATED EGO, 

 farther advanced. a. 

 Umbilical vesicle, b. 

 Am niotic cavity, c. Al- 

 lantois. 



Fig. 209. 



vacant by the rising up of the amniotic fold. This diverticulum 

 is the commencement of the allantois. It is an elongated mem- 

 branous sac, continuous with the posterior portion of the intestine, 

 and containing bloodvessels derived from those 

 of the intestinal circulation. The cavity of the 

 allantois is also continuous with the cavity of 

 the intestine. 



After the amniotic folds have approached and 

 touched each other, as already described, over 

 the back of the foetus, at the amniotic umbilicus, 

 the adjacent surfaces, thus brought in contact, 

 fuse together, so that the cavities of the two 

 folds, coming respectively from front and rear, 

 are separated only by a single membranous par- 

 tition (Fig. 209, c) running from the inner to the 

 outer lamina of the amniotic folds. This parti- 

 tion itself soon after atrophies and disappears ; and the inner and 

 outer laminae become consequently separated from each other. The 

 inner lamina (Fig. 209, a) which remains con- 

 tinuous with the integument of the fcetus, in- 

 closing the body of the embryo in a distinct 

 cavity, is called the amnion (Fig. 210, b), and 

 its cavity is known as the amniotic cavity. 

 The outer lamina of the amniotic fold, on the 

 other hand (Fig. 209, b), recedes farther and 

 farther from the inner, until it comes in con- 

 tact with the original vitelline membrane, still 

 covering the exterior of the egg ; and by con- 

 tinued growth and expansion it at last fuses 

 with the vitelline membrane and unites with 

 its substance, so that the two membranes form 

 but one. This membrane, formed by the fusion 

 and consolidation of two others, constitutes then 

 the external investing membrane of the egg. 



The allantois, during all this time, is increas- 

 ing in size and vascularity. Following the course of the amniotic 

 folds as before, it insinuates itself between them, and of course soon 

 comes in contact with the external investing membrane just de- 

 scribed. It then begins to expand laterally in every direction, 

 enveloping more and more the body of the foetus, and bringing its 

 vessels into contact with the external membrane of the egg. 



FECUNDATED Eoo, 

 with allantois nearly com^ 

 plete. a. Inner lamina of 

 amniotic fold. b. Outer la- 

 inina of ditto, c. Point 

 where the amniotic folds 

 come in contact. The allan- 

 tois is seen penet ating be- 

 tween the inner and outer 

 laminae of the amuiotie 

 folds. 



