Origin of Circulatory Systems
The beginnings of the circulatory apparatus appear very early, since the transport of food stuffs to the region where the new animal is destined to materialize is a primary necessity. In the chick the beginnings of the formation of the circulatory apparatus are as follows. As soon as the fertilized egg has undergone preliminary cleavage, and the potential pioneer cells have arranged themselves into the primary germ layers, certain marginal cells of the spreading splanchnic mesoderm become clumped together, forming so-called blood islands (Fig. 285). These consist of haemopoietic or blood-forming cells surrounded by flat endothelial elements which are destined to become the lining of future capillaries (Fig. 286). The blood islands thus form a halo around the embryo on the surface of the yolk between the endoderm and the splanchnic mesoderm. The developing capillaries that permeate the blood islands coalesce to form eventually a pair of definite blood vessels, the vitelline veins, one on either side. 1 hese lead directly into the growing body of the embryo where they unite into a common trunk which is the beginning of the future heart and ventral aorta (Fig. 287). Anteriorly the ventral aorta splits into two parts from which a pair of vessels, the first aortic loops, extends around onto the dorsal side of the primitive gut and leads into the paired dorsal aortae. The aortae communicate with vitelline arteries which emerge and continue outward to the capillary field of the blood islands. This primary circuit, the first to be established in vertebrate embryos with well-developed yolk, is called the vitelline circulation and has to do with the transportation of food from the yolk sac into the body.
Budding off from the vitelline circuit within the enlarging body of the embryo are secondary trunk lines which extend to and from both anterior and posterior regions of the body. These, together with relics salvaged from the temporary vitelline circuit, later make up the permanent systemic circulation (Fig. 288).
Still a third circulation, the allantoic, is temporarily necessary during the development of higher vertebrates to meet the conditions imposed by embryonic life within an amnion. It consists of a pair of allantoic arteries arising from the posterior region of the aorta, which extend out to supply the temporary saclike respiratory allantois. The returning allantoic veins enter the systemic circulation close to the entrance of the vitelline veins.
In mammals the allantoic arteries and veins become the umbilical arteries and veins that form the respiratory bridge through the placenta between the fetus and mother. The placenta itself is a compound capillary mammalian organ of double derivation, the part from the allantois of the embryo interdigitating into the uterine wall of the mother so that by osmosis there can occur a transfer of materials between the blood of the two. In the earliest known human embryo in which the circulation is described, the allantoic component seems to take precedence over the vitelline circulation, a state of affairs not unexpected since in mammals, as contrasted with reptiles and birds, the yolk sac and the accompanying vitelline blood vessels play a role of ever decreasing importance. The need of a vitelline circuit wanes with the vanishing yolk, while the allantoic circuit becomes useless upon hatching or at birth. Thereafter the systemic circuit takes up its constant functions and maintains them throughout life. The precarious transition from embryonic to permanent circulatory devices involves profound modifications. In mammals, where the umbilical cord is severed at birth, it is very abrupt.