THE BLAST ODERMIC VESICLE IN PRIMATES. 123 



which we can identify as the embryonic shield (compare Fig. 13 and Fig. 68, 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. 69, b.s}, which is the an- 

 lage of the body-stalk. Into this the entoderm has grown in the form of a cylin- 

 drical tubular prolongation, the anlage oi the allantois. As a consequence of the 

 growth of the trophoderm and of the formation of 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 chorionic 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 accompanying extension of the extra-embryonic ccelom into 



Ent 



' FIG. 69. DIAGRAM OF AN EARLY STAGE OF A PRIMATE EMBRYO. 



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

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



the mass of the ectoderm overlying the amniotic cavity (compare Figs. 68, B, and 

 69) until the condition shown in figure 69 is brought about. This stage is known 

 by observation (compare Fig. 80). The amnion, Am, is now completely separated 

 from the chorion, Cho, which forms a relatively large vesicle and consists of a 

 thin layer of mesoderm, and a very thick layer of ectoderm, which has an inner 

 cellular stratum and an outer very much thicker trophodermic stratum. The 

 trophoderm is now very much altered by the appearance of numerous spaces or 

 channels in it which develop so that each of these spaces ends blindly toward the 

 interior of the chorion, buf many of them are open upon the surface of the tropho- 

 derm. As the ovum at this stage is already embedded in the uterine mucosa, the 

 'channels in the trophoderm can receive maternal blood, and such is their original 



