THE EARLY DEVELOPMENT OF THE MAMMAL 429 



but a relatively early condition has been illustrated in Fig. 

 161, D. Here the embryo itself remains connected with the 

 wall of the blastodermic vesicle by the body stalk, described 

 above, and in this respect the human embryo is not a satis- 

 factory example of this type of amnion formation. Usually 

 this attachment represents the remains of a thickening in the 

 wall of the vesicle ("trdger," see above), to which is added 

 later the definitive attachment of the allantois growing out 

 from the embryo through the cavity of the vesicle to the inner 

 surface of the chorion. 



In the human vesicle, however, the amnionic cavity appears 

 directly above the embryo, so that the embryonic disc itself 

 forms its floor, while the body stalk bounds it posteriorly 

 (Figs. 161, 179). At first the roof of the amnionic cavity is 

 simply the trophoblast, but as the cavity enlarges it becomes 

 partly free, below the trophoblast, so that its roof is a separate 

 structure. The attachment of the amnion is around the mar- 

 gin of the embryonic disc (Fig. 162, A); its roof and sides are 

 composed of a thin layer of ectoderm toward the embryo, 

 outside of which is a thin mesodermal layer, for the extra- 

 embryonic mesoderm has already been formed throughout the 

 entire vesicle. A proamnion is therefore lacking in forms whose 

 amnion is developed in this manner. 



As the embryo enlarges and extends in every direction the 

 origin of the amnion appears to be carried ventrally, so that, 

 as in the rabbit, it connects with the embryo just around the 

 umbilicus. At the posterior end, however, it passes around 

 the sides of the body stalk, to its posterior surface, so that this 

 structure finally becomes wrapped in a part of the amnionic 

 membrane (Fig. 179) . In man the yolk stalk and sac, and the 

 allantois, are also bound up in this body stalk or umbilical 

 stalk (Fig. 179). The final disposition and character of the 

 amnion and chorion are thus essentially the same as when 

 formed from folds. 



In man the amnionic cavity grows rapidly and by the third 

 month becomes so large as completely to fill the cavity of the 

 vesicle or "ovum." The exocoelom is thereby entirely obliter- 



