. Cunningham's Text-book of anatomy. Anatomy. Fig. 81. Vena umbilicalis impar Umbilical arteries Vitelline aVteries -Schema of Circulation of ax Embryo, l-35 mm. long, with Six Somites. (After Felix, modified.) The primitive niesantceboids are formed in the wall of the yolk-sac, and there some of them produce erythrocytes; manv, however, migrate into the embryo, where some of them take part in the formation of the walls of the em- bryonic blood-vessels, and others become enclosed in the liver, the lymph glands, and the bone marrow, where they become foci for the formation of blood corpuscles.
. Cunningham's Text-book of anatomy. Anatomy. Fig. 81. Vena umbilicalis impar Umbilical arteries Vitelline aVteries -Schema of Circulation of ax Embryo, l-35 mm. long, with Six Somites. (After Felix, modified.) The primitive niesantceboids are formed in the wall of the yolk-sac, and there some of them produce erythrocytes; manv, however, migrate into the embryo, where some of them take part in the formation of the walls of the em- bryonic blood-vessels, and others become enclosed in the liver, the lymph glands, and the bone marrow, where they become foci for the formation of blood corpuscles. During the first two months the primitive forms of red blood cells predominate. In the second month the sauroid cells in- crease considerably in number, and from the third month the blood plastids become more and more numerous, until, at the eighth month (Minot), the majority of the blood cells are i> blood plastids undergoing conver- sion into blood cor- puscles. At this time the colourless cells are present in a very distinct minority. Formation of the Primitive Blood Vascular System of the Embryo.—The earliest stage of the formation of the heart and blood-vessels in the human subject are not known, but, judging by what occurs in other mammals, it is probable that the first- formed vessels appear in the splanchnic mesoderm of the pericardial region before the embryonic area begins to fold. It is presumed that they are formed by angioblastic cells which have migrated into the embryonic area from the walls of the yolk-sac. From their seat of origin they extend towards the caudal end of the embryonic area, one on either side of the notochord, and from the caudal end of the embryonic region they pass along the body-stalk into the chorion. As the cephalic end of the embryonic area is folded to enclose the fore-gut, the corresponding- parts of the primi- tive arteries are bent into a c-shaped form. The ventral limb of the c, which lies in the dorsal wall of the pericardium and
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1914
