In the mammalian embryo this may be appropriately divided into two systems of different dates; the first, or early circulation, which is confined to the yolk sac; and the second, or later circulation, which passes through the placenta.

The Primitive Heart arises from the splanchnopleural layer of the mesoblast, just at the point where this forms the under wall of the fore part of the alimentary canal. When the formation of the folds of the embryo was described, it was stated that the groove of the cephalic fold tended to grow backward toward the tail end of the embryo. This groove is limited behind by the somatopleural layer of the mesoblast, and posteriorly to this is a cavity formed by the cleavage of the mesoblast, called the pleuroperitoneal cavity. In the early stages of development, the posterior wall of this small cavity is formed by the splanchno-pleural layer of the mesoblast. The heart arises at the point at which the splanchnopleure tends to travel forward to meet the uncleft mesoblast, and thus completes the pleuroperitoneal cavity.

Transverse section through the region of the heart of a rabbit's embryo of nine days old.

Fig. 300. Transverse section through the region of the heart of a rabbit's embryo of nine days old.

(Kdlliker).

ij. Jugular veins, ao. Aorta, ph. Fore-gut. bl. Blastoderm, hp. Body wall reflected in ect. ent. Hypoblast, e'. Prolongation of hypoblast between the two halves of the heart. ah. Outer wall of the heart. p. Cavity of the pericardium, ih. Inner lining of the heart. ect. Epiblast. df. Visceral mesoblast.

Blood Vascular System 319Diagrammatic views of the under surface of an embryo rabbit of nine days and three hours old.

Fig. 301. Diagrammatic views of the under surface of an embryo rabbit of nine days and three hours old, showing the development of the heart. {Allen Thomson).

A. View of entire embryo. B, an enlarged outline of the heart of A. C, a later stage of the development of B. hh. Ununited heart, aa. Aortae. VV. Vitelline veins.

The heart consists at first of a single cylinder, which in the human embryo is probably formed by the coalescence of two primary tubes. At first it has no distinct cavity, but soon the cells of the mesoblast within the mass forming the heart become transformed into blood corpuscles, and thus it is hollowed out. A layer of endothelial cells line the cavity, and become the endocardium.

Blood Vascular System 321

Fig. 302. Fig. 303.

Human embryo of about three weeks. {Allen Thomson).

Human embryo of about three weeks. {Allen Thomson).

uv. Yolk sac.

al. Allantois.

am. Amnion.

ae. Anterior extremity.

pe. Posterior extremity.