The upper or somatic layer of the mesoblast becomes very closely con nected with the epiblast, the two together practically forming a single layer, which is spoken of as the somatopleure. Similarly the lower or splanchnic layer of the mesoblast becomes intimately connected with the hypoblast, with which it forms what is practically a single layer—the splanchnopleure.
The Body Cavity.—In the embryo itself tie somatopleure forms the lateral and ventral portions of the body wall, while the splanchnopleure forms the similar portions of the wall of the alimentary canal. The space between somatopleure and splanchnopleure—i.e., the actual split between the two layers of the mesoblast—becomes the pleuro-peritoneal or body of the embryo. These relations will become more evident from an inspection of Fig. 99.
The Amnion.—Outside the embryo the somatopleure rises up so as to form a low wall surrounding the embryonic area. This wall, which re ceives the name of amnion, rapidly increases in height (Fig. 98. 2), and the folds forming it arch over the back of the embryo so as partially to cover it. A little later the folds meet one another along the middle line of the back, and so completely cover the embryo (Fig. 98, 3). At this period we have an inner layer of the amnion (Fig. 98, 3, a in) closely in vesting the embryo, and an outer layer (Fig. 98, 3, 8 h) lying immediately beneath the zona pellucida and continuous below with the outer wall of the yolk-sac, d 8.
A little later still, the two layers of the amnion coalesce along their line of meeting above the back of the embryo, and the inner layer becomes completely separated from the outer layer (Fig. 99). At the same time, owing to the splitting of the mesoblast extending all round the yolk-sac, the outer layer of the amnion becomes completely separated from the yolk-sac, and now forms a thin lining membrane closely applied to the inner surface of the zona pellucida.
The condition of things at this stage is shown in Figs. 98, 4, and 99.
The outer layer of the amnion, s h in Fig. 98, 4, and 8 z in Fig. 99, lining the zona pellucida, sometimes receives the name false amnion, but is better called the subzonal membrane; the inner layer a ni, which forms an investment over the back of the embryo, but at some little distance from it, and which is clearly, from its mode of formation as a fold of so matopleure, continuous with the body wall of the embryo, is spoken of as the true amnion, or simply as the amnion.
It will further be evident from the figures that the space between the two layers of the amnion, r in Fig. 98, 4, being part of the space formed by the splitting of the mesoblast, is continuous with the body-cavity of the embryo.
It is customary to speak of the space between the inner layer of the amnion and the embryo, A C in Fig. 99, which is really not between the layers of the amnion at all, as the amnionic cavity.