. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Internal Transportation 35* 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 d
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Internal Transportation 35* 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 Common Cardinal Vein Dorsal Aorta Jugular Vein Aortic Arches Posterior Cardinal Vein / Liver Diverticulum „Subintestinal Vein -Caudal Vein. Mouth / Heart Ventral Aorta ,«-' Omphalomesenteric Vitelline Vein Vitelline Artery Fig. 288. Diagram of embryonic circulation in a large-yolked vertebrate. (After Kingsley.) 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 con- stant functions and maintains them throughout life. The precarious tran- sition from embryonic to permanent circulatory devices involves profound modifications. In mammals, where the umbilical cord is severed at birth, it is very abrupt. V. THE HEART 1. In General The vertebrate heart, which is essentially a modified blood vessel half artery and half vein, consists of two kinds of chambers, a thin-walled venous receiving chamber, where the returning blood collects, and a thick-walled arterial muscular forwarding chamber, separated from the former by valves. Please note that these images are extracted from sca
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