292 EMBRYOLOGY 



hormones at birth from the maternal blood allows the pituitary to secrete 

 more prolactin, which is necessary for the actual secretion of milk from the 

 mammary gland. Also the pituitary resumes its production of FSH and LH 

 (ICSH) for the normal ovulation cycle in the ovary. 



Finally, after the withdrawal of estrogens and progesterone the uterus 

 returns to its normal, nonpregnant size. This regression may take several 

 weeks. After birth the uterus contracts, the blood vessels constrict, and the 

 decrease in size from about 1000 g. of pregnant uterus to about 60 g. of non- 

 pregnant uterus begins. Pituitrin injections aid in the contraction of the 

 uterus after birth. In Caesarian births pituitrin is injected to contract the 

 uterus. 



During and after birth vitamin K, a factor in the rapid clotting of blood, 

 is of practical importance. Vitamin K is normally produced by bacteria in 

 the intestine and, since there are no bacteria in the fetal intestine, the vita- 

 min K content of the newborn child is very low. To insure rapid clotting of 

 the blood of the child in the event of hemorrhages during birth, vitamin K 

 may be injected into the mother. Furthermore, the vitamin may be injected 

 if any operations are to be performed in the child after birth. 



The newborn child may have an acute anemia due to the destruction of 

 blood cells by a substance present in the maternal blood stream. This con- 

 dition, which varies in intensity, is termed erythroblastosis and has been 

 traced to a gene, Rh. This gene is dominant and, when present in an 

 individual, gives rise to Rh-positive blood. Rh-positive blood is agglutinated 

 by the blood of a rabbit which has received injections of the blood of a 

 rhesus monkey. Hence, the use of the term Rh. The rhesus monkey blood 

 causes the formation of an anti-Rh substance in the rabbit blood. The anti-Rh 

 substance then can agglutinate Rh-positive blood cells. If both parents are 

 Rh-negative or both Rh-positive no harm can come to the offspring. Or, as 

 we shall see, if the mother is Rh-positive and the father Rh-negative, the 

 blood of the fetus and mother are compatible. But when the mother is Rh- 

 negative and the father Rh-positive difficulty may arise. However, there is 

 much variation in the expression of these genes, and erythroblastosis does 

 not always arise when expected. 



The mechanism appears to be as follows. From the union between Rh- 

 negative mother and Rh-positive father an Rh-positive fetus develops with 

 a substance in its blood cells which, upon entering the maternal blood stream 

 through the placenta, builds up an antibody, or anti-Rh substance. The ma- 



