The development of the human body; a manual of human embryology . s begun to beat before theconnection with the vessels is made, so that when it ismade, the circulation is at once established. Before,however, the vascularization reaches the embryo some of the canals begin to en-large (Fig. 122, A), pro-ducing arteries and veins,the rest of the networkforming capillaries unit-ing these two sets ofvessels, and, this processcontinuing, there areeventually differentiateda single omphalo-mesen-teric (vitelline) artery andtwo omphalo - mesenteric(?vitelline) veins (Fig. 122,B). In the human embryoth
The development of the human body; a manual of human embryology . s begun to beat before theconnection with the vessels is made, so that when it ismade, the circulation is at once established. Before,however, the vascularization reaches the embryo some of the canals begin to en-large (Fig. 122, A), pro-ducing arteries and veins,the rest of the networkforming capillaries unit-ing these two sets ofvessels, and, this processcontinuing, there areeventually differentiateda single omphalo-mesen-teric (vitelline) artery andtwo omphalo - mesenteric(?vitelline) veins (Fig. 122,B). In the human embryothe small size of theyolk-sac permits of theextension of the vascu-lar area over its entiresurface at an early pe-riod, and this conditionhas already been reachedin the earliest stagesknown and consequentlyno sinus terminalis suchas occurs in the rabbit isvisible. Otherwise the conditions are probably similar towhat has been described above, the first circulation de-veloped being associated with the Formation of the Blood.—The erythrocytes, which. Fig. 121.—Surface View op a Por-tion of the Area Vasculosa ofa Chick. The vascular network is represented bythe shaded portion. Bi, Blood-island;Si, substance-island.—(Disse.) THE BLOOD. 243 are the first blood-eorpuscles, are all nucleated and are fora time the only cells occurring in the blood, though laterother cells, arising in tissues exterior to the blood-vessels,make their way into the vessels, forming leukocytes. Fromtheir very first formation then the red (erythrocytes) andwhite (leukocytes) blood-corpuscles have a different ori-gin, and they remain distinct throughout life, one formnever becoming converted into the other. *<v75
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
