48 GENERAL EMBRYOLOGY. 
of description it is usual to divide the amnion fold into four parts, the cephalic, the 
caudal, and the two lateral amnion folds; these, however, are all continuous with 
one another. 
The inner part of the fold, which is formed from the amniotic area, is termed 
the true amnion, and the outer part, formed from the chorionic area, the false 
amnion. The latter term is, however, synonymous with chorion, and as it is mis- 
leading, it should be avoided. 
As the amnion is formed from the amniotic area of the blastoderm after the 
extension of the ccelom, it must consist, as previously mentioned, of ectoderm and 
somatic mesoderm, and as the surface of the amniotic area is reversed during the 
formation of the amnion folds, it is obvious that in the fully-formed amnion the 
ectodermal layer is internal and the somatic mesoderm external. 
After the amnion is completed its cavity is distended with fluid. As it expands 
it gradually obliterates the extra-embryonic part of the celomic cavity, and finally 
its outer surface, of somatic mesoderin, comes into contact and fuses with the somatic 
mesoderm on the inner surface of the chorion. At this period the cavities in the 
ovum are the amniotic cavity, the remains of the yolk-sac, and those portions of the 
original blastodermic and ccelomic spaces which have been included in the embryo. 
In the human ovum, when the amnion folds unite and the true amnion separates 
from the chorion, the embryo and its enclosmg amnion would be free within the 
cavity of the chorion, or extra-embryonic ccelom, were it not that a very short cord 
of somatic mesoderm and ectoderm, the body-stalk, connects the posterior end of the 
embryo with the somatic mesoderm on the inner surface of the chorion. 
The Body-Stalk.—To thoroughly understand how this union is effected in the 
human ovum, and to comprehend the nature of the body-stalk, it is necessary to 
refer to some striking peculiarities which are to be observed in the earlier stages 
in the development of the human embryo. When segmentation is completed, and 
the morula is converted into a blastula by the appearance of a cavity in its interior, 
the human ovum consists of an outer layer, the ectoderm, and an inner cell mass 
(Figs. 10 and 37). The latter, however, which is attached to a small area of the 
ectoderm, does not, as in many mammals, extend itself by migration round the inner 
surface of that layer, and so transform the unilaminar into a bilaminar blastoderm and 
convert the cavity of the blastula into the blastodermic cavity. The sequence of 
events is different: a cavity or space appears in the inner cell mass itself (Figs. 
38, 39,and 41), and this expanding rapidly, is ultimately converted into the yolk-sae 
and the alimentary canal of the embryo; it corresponds, therefore, with the blasto- 
dermic cavity of other mammals. 
Thus the entoderm, though derived from the inner cell-mass, never lines the 
inner surface of the ectoderm except in the embryonic area, for soon after the 
appearance of the cavity of the inner cell-mass the mesoderm grows rapidly from 
the primitive streak and extends, not in a single layer, as in the majority of 
mammals, but as two layers, one over the outer surface of the entoderm, the 
splanchnic layer, and the other, the somatic layer, over the inner surface of the 
ectoderm. The cavity of the blastula is thus ultimately enclosed between the 
somatic and splanchnic layers of the mesoderm, and so becomes converted into the 
coelomic space (Fig. 39). 
As the mesoderm extends, the several areas of the blastoderm are differentiated 
as in other mammals, but the embryonic and amniotic areas remain of relatively 
small size. The separation of the amnion from the chorion is effected at a very 
early period, before the folding off of the embryo has commenced, but the somatic 
mesoderm growing from the posterior end of the embryonic area still retains its 
connexion with the similar layer on the inner surface of the chorion, and it forms 
a short, and for a time a broad stalk which unites the embryo, and consequently 
the amnion and the blastodermic cavity, with which the embryo is connected, to the 
chorion (Fig. 39). In addition to forming a bond of union between the embryo 
and the chorion the mesodermal stalk conducts blood-vessels from the embryo to 
the chorion, and more especially to its placental part. 
At an early period a pouch-like diverticulum projects from the posterior part 
of the entodermal sac. This is the allantoic diverticulum; it les beneath the 
