. Principles of modern biology. Biology. 392 - Multicellular Animals, Especially Man. Fig. 21-15. Human embryo ( mm long) enclosed in the embryonic mem- branes. The yolk sac lies below, connected with the embryo by the umbilical cord. The embryo almost completely fills the amniotic cavity, which is bounded by the plainly visible amniotic membrane. The outermost membrane is the chorion, from which the tufted chorionic villi (embryonic part of the placenta) protrude. Gill slits appear on the side of the neck, and the mesodermal somites show along the dorsal margin of the trunk. The limbs are
. Principles of modern biology. Biology. 392 - Multicellular Animals, Especially Man. Fig. 21-15. Human embryo ( mm long) enclosed in the embryonic mem- branes. The yolk sac lies below, connected with the embryo by the umbilical cord. The embryo almost completely fills the amniotic cavity, which is bounded by the plainly visible amniotic membrane. The outermost membrane is the chorion, from which the tufted chorionic villi (embryonic part of the placenta) protrude. Gill slits appear on the side of the neck, and the mesodermal somites show along the dorsal margin of the trunk. The limbs are present, but no fingers or toes have yet been formed. (Courtesy of W. Chesterman, Oxford University.) which penetrate deeply into the uterine wall (Fig. 21-15). This region, where the tissues of the chorion and uterus are intermingled, is the placenta, which permits exchanges to occur between the blood of the fetus and that of the mother. The chorionic villi are richly supplied with blood capillaries, de- rived from the umbilical artery. This large artery extends out from the embryo along the stalk of the allantois, and the chorionic capillaries are drained by the umbilical vein, which returns to the embryo parallel to the umbilical artery (Fig. 21-16). The uterine wall, in the region of the chorionic villi, is also highly vascularized, and this tissue con- stitutes the maternal part of the placenta. The central part of the placenta has the form of a circular disc, embedded in the uterine wall at the site of implantation (Fig. 21-16), but some placental tissue tends to. 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 Marsland, Douglas, 1899-. New York, Holt, Rinehart and Winston
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