The Yolk Sac is that part of the blastoderm which grows and envelops the yolk, which previously was only surrounded by the vitelline membrane. After the mesoblast has split into two layers, the splanchnopleure becomes bent inward at a point some distance from its origin, carrying with it the hypoblast. By this curve an upper constricted canal is differentiated from the large lower cavity. This upper canal becomes eventually the alimentary tract, the lower cavity the yolk sac, while the constricted portion leading from one to the other is the canal leading from the intestine to the yolk, called the ductus vitello-intestinalis.

At first the splanchnopleure encloses only the upper part of the yolk, but as development proceeds it grows around, and at last completely encircles it. The yolk sac is thus derived from the splanchnopleural layer of the mesoblast, and its lining hypoblast.

The yolk is continually used up for the nutrition of the embryo, and its covering shrinks in size, becoming smaller with the growth of the foetus, until eventually it forms but a shriveled protrusion from the intestine, lying in the umbilical cord.

Diagrammatic longitudinal section of a chick on the fourth day.

Fig. 273. Diagrammatic longitudinal section of a chick on the fourth day. {Allen Thomson).

ep. Epiblast. hy. Hypoblast, sm. Somatopleure. vm. Splanchnopleure. af,pf. Folds of the amnion, pp. Pleuroperitoneal cavity, am. Cavity of amnion, al. Allantois. a. Position of the future anus. h. Heart, i. Intestine, vi. Vitelline duct. ys. Yolk, s. Foregut. m. Position of the mouth, me. The mesentery.

The importance of the yolk sac differs largely in mammalia and birds. In man it is not highly developed, as its place is early supplied by the placenta. In birds it develops to a much higher degree, being the seat of a special circulation, which carries nourishment from the yolk to the chick. The vessels are developed in the mesoblastic portion of the membrane, and are called the omphalo-mesenteric vessels, which convey blood to and from the primitive heart.