. Embryology. Embryology; Embryology; genealogy. 276 EMBRYOLOGY coelom. The two heart rudiments have fused to form a single tube of endocar- dium surrounded by a thicker wall of myocardium. At about this time, or per- haps even earlier, the heart begins to beat. Some blood cells formed by the blood islands of the yolk sac are present, and circulation through the yolk sac and the body stalk is complete. Up until this time the embryo has had no nutrition through the placenta and has had to rely on diffusion of products from the uterus (uterine milk) and on products of disintegration of cells of
. Embryology. Embryology; Embryology; genealogy. 276 EMBRYOLOGY coelom. The two heart rudiments have fused to form a single tube of endocar- dium surrounded by a thicker wall of myocardium. At about this time, or per- haps even earlier, the heart begins to beat. Some blood cells formed by the blood islands of the yolk sac are present, and circulation through the yolk sac and the body stalk is complete. Up until this time the embryo has had no nutrition through the placenta and has had to rely on diffusion of products from the uterus (uterine milk) and on products of disintegration of cells of the wall of the uterus. These cells break down during implantation to form a debris which is used as food by the growing embryo. With the establishment of a complete circulation at the third week, the embryo derives its food through the placenta, as outlined in Figure 168. Many substances pass through the placental barrier. In general, only the smaller molecules penetrate the three layers of cells that separate the fetal and maternal blood. However, some antibodies of large molecular size do pass from mother to fetus and confer immunity on the latter. Although the acquisition of some antibodies is beneficial for the newborn child, other in- jurious large molecules also can get through the placenta—for example, the anti-Rh factor, which produces erythroblastosis. Furthermore, if the mother ORAL CAVITY BULBUS. YOLK SAC :%-OTOCYST ARM BUD BODY REGION LEG BUD BODY STALK Fig. 185. A human embryo about 26 days old. (From Julius Kollman, Handatlas der Entwickhtngsgeschichte des Memchen. Verlag von Gustav Fischer, Jena.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Barth, Lester G. (Lester George), 1905-1979. New York, Dryden Press
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