The mesoblast around the embryo becomes thickened, and is split into two distinct layers; this cleavage is at first confined to the neighborhood of the embryo, but gradually spreads over the whole blastoderm.
The upper of these two layers of the blastoderm receives the name of the somatopleure, and is engaged in the formation of the body walls of the embryo and the amnion. The lower one is called the splanchnopleure, and forms the walls of the alimentary canal, the allantois, and the yolk sac. The space intervening between these layers is called the pleuroperitoneal cavity. At a point in front of the cephalic fold, an upward projection of somatopleure takes place, conveying with it the overlying epiblast. Along the sides of the embryo and behind the caudal fold, projections of the somatopleural mesoblast and epiblast also occur. Thus folds are developed consisting of somatopleural mesoblast and of epiblast, which tend to grow upward and meet over the back of the embryo. These are the amniotic folds, and each presents two surfaces, one looking toward the embryo and the other toward the vitelline membrane. As they meet over the back of the embryo the folds become fused, the membranes looking toward the embryo joining to form the amnion proper, while those next the vitelline membrane unite to form the false amnion, which, separating from the amnion proper, retires toward the vitelline membrane, with which it unites to form the primitive chorion.
The proper amnion then is a sac formed of an outer layer derived from the mesoblast and an inner layer derived from the epiblast. The false amnion likewise consists of mesoblast and epiblast, but here the epiblast is external. The amnion proper is continuous with the skin of the embryo, and when the foetus is mature, the connection may be traced by the umbilical cord, round which it forms a sheath continuous with the skin at the umbilicus. This membranous sac enlarges, and in mammalia eventually becomes the large bag of liquid which contains the foetus. The amniotic liquid is of low specific gravity, consisting mainly of water containing traces of nitrogenous matter, phosphates and chlorides.
Fig. 272 and the following two wood-cuts are diagrammatic views of sections through the developing ovum, showing the formation of the membranes of the chick. (Foster and Balfour.) A, B, C, D, E and F are vertical sections in the long axis of the embryo at different periods, showing the stages of development of the amnion and of the yolk sac. I, II, III and IV are transverse sections at about the same stages of development, i, ii and iii give only the posterior part of the longitudinal section, to show three stages in the formation of the allantois.
e. Embryo, y. Yolk. pp. Pleuroperitoneal fissure, vt. Vitelline membrane.
af. Amniotic fold. al. Allantois.
It contains albumin and some other nitrogenous constituents, and a minute quantity of urea, which is thought to be derived from the foetal kidneys.
Fig. 272. A. e. Embryo, a. Amnion, a'. Alimentary canal, vt. Vitelline membrane, af. Amniotic fold. ac. Amniotic cavity, y. Yolk. al. Allantois.
This fluid preserves the child from the effects of any jolts or jars caused by the movements of the mother, and similarly protects the uterus of the mother by acting as a buffer between the foetus and the uterine wall. Before delivery it helps to dilate the os uteri, so that when the amnion is ruptured the head of the foetus occupies the opening which has been gradually made by the fluid wedge. The outer part of the amniotic membrane, derived from the mesoblast, is of a tougher character than the inner epithelial layer, possesses muscular fibre and is capable of rhythmical contractions.
Fig. 272. B. Diagrammatic sections of an embryo, showing the destiny of the yolk sac, ys. vt.Vitelline membrane. pp. Pleuroperitoneal cavity, ac. Cavity of the amnion, a. Amnion. a'. Alimentary canal, ys. Yolk sac.