THE ORIGIN OF THE AMNION. 117 



the mass of thickened ectoderm overlying the yolk-sac there has appeared a 

 cavity, — the future amniotic cavity, — which is, of course, entirely surrounded by 

 ectoderm. The portion of the ectoderm on the under side of this cavity consists 

 of a single layer of cells which by assuming a cylindrical form constitute the 

 thickened area which we can identify as the embryonic shield (compare Fig. i8 

 and Fig. 53, B). The solid mass of ectoderm above the amniotic cavity is 

 later to form a part of the amnion and part of the chorion. At the posterior 

 end of the embryo there appears a considerable accumulation of mesoderm (Fig. 

 54, h. s), which is the anlage of the body-stalk. Into this the entoderm has 

 grown in the form of a cylindrical tubular prolongation, the anlage of the allan- 

 tois. As a consequence of the growth of the trophoblast and of the formation of 



Emb. 



Ent. 



Fig. 54. — Diagram of an Early Stage of a Primate Embryo. 



All, Allantois. Am, Amnion, b.s. Body-stalk. Cho, Chorion. Emb, Embryo. Ent, Entoderm. In, 



Entodermal cavity of embryo. Vi, Villi of chorion. Yk, Yolk-sac. 



the amniotic cavity, the embryo or embryonic shield, Emb, together with the 

 yolk-sac, Yk, attached to it, has been forced down into the interior of the cho- 

 rionic vesicle. This phenomenon is very marked in certain rodents and leads to 

 the so-called inversion of the germ-layers. In the next stage the amnion is 

 formed. This is accomplished by the penetration of the mesoderm with accom- 

 panying extension of the extra-embryonic coelom into the mass of the ectoderm 

 overlying the amniotic cavity (compare Figs. 53, B, and 54) until the condition 

 shown in figure 54 is brought about. This is the stage known by observation. 



