GENERAL ZOOLOGY 



instances is tliere any evidence of transfer of disease or poisons from the 

 mother to the embryo in the uterus. The microorganism causing syphilis can 

 pass the placenta and produce the disease in the fetus, as the human embryo 

 is called after the third month. If a woman has German measles during the 

 first 3 months of pregnancy, the causative agent passes the placenta and brings 

 about severe effects in the embryo. 



A striking illustration of another of the few disadvantages inherent in the 

 intimate association of the embryo with its mother has been discovered. This 

 is the origin of a very serious disease (fetal erythroblastosis) which reduces 

 the number of red blood cells in the fetus during late pregnancy. It has been 

 found that human red cells may have an antigen called the Rh (D) substance 

 in addition to A and B (p. 65). Whether or not these antigens are present 

 depends on the heredity of the individual (p. 199). Individuals who lack 

 the Rh antigen do not normally contain an antibody for it; only 15 per cent 

 of the general population lack the Rh antigen. If a man has the Rh sub- 

 stance and his wife lacks it, at least half and perhaps all their children will 

 produce red cells containing the Rh substance. It appears certain that 

 enough Rh-containing red blood cells, or fragments of them, pass from the 

 blood vessels of the fetus to the blood of the mother in the placenta to 

 stimulate the production of Rh antibodies by the mother. She immunizes 

 herself against this foreign protein, the Rh substance. These antibodies, 

 when they have become abundant enough, begin to diffuse back through the 

 placenta into the blood of the fetus. Here they result in the destruction 

 of the fetal red blood cells and, consequently, in excessi\'e drain on the 

 regions forming red cells, usually with fatal results. The anti-Rh substances 

 persi-st in the blood of such a woman and, if her husband is homozygous 

 (p. 186) for Rh, make it almost impossible for her to bear a second living 

 child. Ervthroblastosis results in the death of the fetus or newborn infant 

 in slightly more than 2 per cent of the pregnancies in the white population 

 of this country. If a partially Jaundiced child born of Rh-incompatible 

 parents is given an exchange blood transfusion within 24 hours after birth, it 

 may be able to recover from the effects of the excessive destruction, in utero, 

 of its red blood cells. In an exchange transfusion, matched Rh-negative 

 donor blood is passed, over a period of 2 to 3 hours, into the infant's cir- 

 culatory system through an umbilical vessel as the infant's blood is with- 

 drawn. Such a procedure provides the red blood cells necessary for oxvgen 

 transport until the infant's own supply of red cells is built up in its blood- 

 forming organs which have not been permanently damaged by the severe 

 drain on them. 



The embryonic membranes are not permanent structures. When the rep- 

 tile or bird hatches, the embryonic membranes are left in the shell. After 

 the birth of a mammal its embryonic membranes are expelled from the uterus 

 of the mother. The development of such structures by reptiles, birds, and 

 mammals has made them independent of an aquatic environment during 

 development. Amphibians that are terrestrial as adults must undergo 



164 



