. Principles of modern biology. Biology. 326 - Multicellular Animals, Especially Man positive and complications involving the Rh factor do not arise from intermarriages within this majority group. After the first child, how- ever, the offspring of an Rh positive father and an Rh negative mother are endangered by the possibility that a reaction will occur between the blood of the mother and that of the fetus. In this case, the fetus always inherits an Rh positive blood, and during the first pregnancy the fetal antigen stimu- lates the production of Rh antibody in the maternal plasma. The produc
. Principles of modern biology. Biology. 326 - Multicellular Animals, Especially Man positive and complications involving the Rh factor do not arise from intermarriages within this majority group. After the first child, how- ever, the offspring of an Rh positive father and an Rh negative mother are endangered by the possibility that a reaction will occur between the blood of the mother and that of the fetus. In this case, the fetus always inherits an Rh positive blood, and during the first pregnancy the fetal antigen stimu- lates the production of Rh antibody in the maternal plasma. The production of Rh antibody by the mother is not rapid enough to produce much reaction in the first child, but during another pregnancy a widespread agglutination and subsequent destruction of the red cells may occur in the fetal blood. This condition is recognized as erythro- blastosis fetalis, a drastic type of anemia that may be fatal to the embryo before de- livery or to the child soon after. THE VERTEBRATE HEART Embryologically the vertebrate heart rep- resents a highly modified blood vessel with thick contractile walls. In man and other vertebrates, this special vessel develops in the mid-ventral region of the embryo, just behind the gill slits. The embryonic heart soon differentiates into two muscular cham- bers—the auricle and ventricle—which then begin to pulsate rhythmically (Fig. 17-7). The auricle has a thinner wall than the ventricle. Each time it relaxes the auricle collects blood from the sinus venosus (Fig. 17-7); when it is filled, the auricle contracts, forcing the blood into the ventricle. Then the thick-walled ventricle contracts, forcing the blood to flow at high pressure, out into the arteries of the body. Valves, situated at the entrance and exit points of the auricle and ventricle, prevent any appreciable back- flow; practically all the force of the contract- ing heart goes to the propulsion of the blood in a forward direction. The simple two-chambered heart
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