Quain's elements of anatomy . ior caixlinal vein; ,inferior ; the arteries are shaded black, the veins are in outline ; the embryo seen fromits lower or left side is only faintly indicated. vascular elements are situated in the deepest part of the visceral plate ofthe mesoblast, which was thence distinguished by Pander as the vascularlayer of the blastoderm. At a later period it is probable that blood-vessels may arise tlnoughout the v/hole of the mesoblast, both in its?visceral and parietal divisions. SoG THE BLOOD-TASCULAR SYSTEM. Tho simultaneous origin of the blood-vessels and heart, p
Quain's elements of anatomy . ior caixlinal vein; ,inferior ; the arteries are shaded black, the veins are in outline ; the embryo seen fromits lower or left side is only faintly indicated. vascular elements are situated in the deepest part of the visceral plate ofthe mesoblast, which was thence distinguished by Pander as the vascularlayer of the blastoderm. At a later period it is probable that blood-vessels may arise tlnoughout the v/hole of the mesoblast, both in its?visceral and parietal divisions. SoG THE BLOOD-TASCULAR SYSTEM. Tho simultaneous origin of the blood-vessels and heart, partly withinthe body of the embryo and partly in the vascular area of the yolk,appears to be universal tlnoughout the vertebrata, and is only modifiedin form in a few instances, as in some of the amphibia, by theundeveloped condition of the yolk sac. In the human ovum the sameappears to be the first condition of the blood-vascular system, and assoon as before the fourteenth day from conception a simple tubular Fi£c. 764. A. Fig. 764. DiAGRAMJIATIC VIE^yS OF TWO SECTIONS THROTJGH THE REGION OP THE HIND- I3RAIN OF AN EMBRYO-CHICK OP ABOUT 36 HOURS, ILLUSTRATING THE FORMATION OF THE HEART. (From Balfour.) In A, wliicli is farthest forward, the parts of tke heart liave coalesced in tliomiddle ; in B, which is farther back, they are still widely separate. hh, hind-brain ; nc, notochord ; E, epiblast ; so, body-wall ; sj:), visceral wall ; d,alimentary canal; hy, hypoblast ; hz, heart; of, vitelline veins. heart with incoming and outgoing vessels are formed in the body of theembryo, while the surface of the yolk or umbilical vesicle is occuijied byramified vessels and their capillary network. The phenomena of vascular and blood formation have been observedAvith gTeat minuteness and care, as they occur in the egg of the commonfowl in the first half of the second day of incubation, and a sufficientamount of observations have been made in mammals and other animals THE HEART. 857 to pro
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Keywords: ., boo, bookcentury1800, booksubjectanatomy, booksubjecthumananatomy
