THE EARLY DEVELOPMENT OF THE MAMMAL 427 



above the level of the surface of the embryo forming the rudi- 

 ment of the head-fold of the amnion, which is thus composed 

 at first of the ectodermal and endodermal wall of this part of 

 the vesicle. The posterior and lateral folds rapidly come to- 

 gether, close above the embryo, in the anterior direction (Fig. 

 177). Finally all four of the folds fuse together in front of the 

 middle of the embryo. The region of their fusion, the sero- 

 amnionic connection, is a small knot, quite in contrast to the 

 elongated seam of the chick; it should also be noted that the 

 direction of the closure of the folds is just' the reverse of what 

 it is in the chick, for here the tail-fold, rather than the head- 

 fold, grows the more rapidly. 



Following the complete fusion of the amnionic folds, occurs 

 the separation of the inner and outer layers of the folds, thus 

 establishing (1) an outer membrane, really an extension over 

 the embryo of the wall of the blastodermic vesicle, known as the 

 chorion; (2) an inner membrane, the amnion, separated from 

 the embryo itself by (3) the amnionic cavity; and (4) an ex- 

 tension completely around the dorsal and lateral sides of the 

 embryo, of the exoccelom (Fig. 176). 



From the relation of these folds (Fig. 178), it is clear that the 

 chorion is ectodermal superficially, lined with extra-embryonic 

 somatic mesoderm, while the amnion is ectodermal internally 

 with its somatic mesodermal layer turned away from the em- 

 bryo. The exoccelom is of course entirely lined with meso- 

 derm, while the amnionic cavity is wholly lined with ectoderm, 

 embryonic and extra-embryonic. The amnion is a thin, semi- 

 transparent membrane, while the chorion is thicker and quite 

 opaque. The amnion is non-vascular, while the chorion is, in 

 the higher Mammals, often richly supplied with blood vessels. 



As the embryo enlarges, the attachment of the amnion, and 

 therefore the region where the amnionic ectoderm becomes 

 continuous with the embryonic ectoderm, remains restricted to 

 the region just around the origin of the yolk stalk and allantois 

 or umbilicus (Fig. 176, B, C). 



The proamnion finally becomes invaded by mesoderm, which 

 has from a very early stage been present in front of the pro- 



