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 of the 

 early embryo is essentially similar to the fully 

 developed heart of lower (fishlike) verte- 

 brates, in which gills serve as the respiratory 

 organs. Lung-breathing vertebrates, however, 

 except for the Amphibia, have developed a 

 four-chambered heart, with a separate auri- 

 cle and ventricle to pump blood through the 

 lungs (Figs. 17-8). Regardless of type, how- 

 ever, the heart is essentially a hollow muscu- 

 lar chamber that contracts repeatedly, forc- 

 ing the blood to circulate. 



Primitive organisms may lack a definite 

 heart, or strictly localized pumping organ. 

 The earthworm, for example, possesses a 

 dorsal blood vessel, which is contractile all 

 throughout the length of the body (Fig. 17 

 9). This elongate blood vessel, together with 

 the five pairs of vessels that encircle the 

 esophagus of the earthworm (Fig. 17-9), main- 

 tains a steady series of contractions that 

 propel the blood. The flow is entirely in one 



LARGE VEINS- 



VENTRICLE 



AORTA 



SINUS 

 VENOSUS 



AURICLE 



Fig. 17-7. An eorly stage in the development of the heart. 



