DEVELOPMENT FROM 20 TO 38 WEEKS 287 



effect of the pressure exerted by uterine contractions. Just after birth the rate 

 jumps up to about 170 or more beats per minute, which may be the result of 

 nervous stimulation. Shortly afterward, the heart rate settles down to about 

 110 beats per minute. 



The more rapid heart beat of the small fetus as compared with the larger 

 newborn recalls the very rapid heartbeat in small mammals, such as the bat 

 and the mouse, as compared with large mammals, such as the elephant. In 

 these cases it is known that the rate of metabolism of small mammals is 

 higher than that of large ones. 



During the fifth month hair and nails develop in the skin. Hair becomes 

 visible about the fifth or sixth month and forms a dense covering over the 

 body during the seventh month. This fetal hair is shed during very late 

 pregnancy and collects on the surface of the fetus in a fatty layer secreted by 

 the sebaceous glands of the skin. This layer of vemtx caseosa is present at 

 birth and is removed. The nails begin formation very early — around the fifth 

 month — and are fully formed before birth. 



Pigmentation of the skin is usually very light, and pigment forms after 

 birth for several months. Eye color does not develop until after birth, and 

 may not be complete for a few months. On the other hand, the pigmentation 

 of the retina occurs very early in the embryo. The mammary glands begin 

 development as early as six weeks and form a mammary ridge on either side 

 of the embryo, extending between the arms and legs. Differentiation of the 

 two glands occurs at the anterior region of the mammary ridge although in 

 rare cases extra mammary glands may develop from the more posterior parts 

 of the ridge. At birth a small amount of secretion exudes from the glands of 

 both sexes. 



Although the external genitalia are characteristically male or female at 

 about twelve weeks, the testes remain in the body cavity until about seven 

 months, at which time the migration into the scrotum begins. At about nine 

 months the descent of the testes is completed. Sometimes, for one reason or 

 another, the testes fail to descend into the scrotum. Under these conditions 

 the testes are sterile, owing to the higher temperature within the abdomen. If 

 the fault is lack of hormone, the descent of the testes may be brought about 

 by the injection of gonadotropic hormones of the pituitary. Normal descent 

 of the testes, as well as some other sex differentiation, is under the control of 

 the fetal hypophysis. 



During late fetal life many glands are capable of function. The pituitary 



