HUMAN REPRODUCTION 253 



mones and antibodies from the mother's blood diffuse into that of the 

 embryo, and carbon dioxide, nitrogenous wastes and hormones pass from 

 the embryonic into the maternal circulation. 



The details of human embryonic development do not differ greatly 

 from those of other mammals, and it would be beyond the province of 

 this book to discuss them at any length. All of the special vertebrate 

 structures (somites, notochord, hollow dorsal neural tube, gill pouches, 

 and limb buds) treated in the preceding chapter arise in man in the way 

 there described. Nevertheless, because of our natural interest in all 

 aspects of human biology, we have included photographs of human 

 embryos in eight stages of development, with some explanatory notes, 

 on pages 250-251. 



At full term the fetus (as the embryo is called when it begins to be 

 recognizably human) is enclosed within the greatly enlarged uterus and 

 lies inside a fluid-filled sac composed of the chorion on the outside and 

 the amnion on the inside. One side of this sac forms the placenta, to which 

 the fetus is attached by the umbilical cord. Birth is accomplished by the 

 periodic contractions of the powerful smooth muscles of the uterus, aided 

 by the voluntary muscles of the abdominal walls. During the first stage 

 of the process contractions of the uterus force the amniotic sac downward 

 into the neck of the uterus, which gradually relaxes and expands. Finally 

 the sac breaks, and the baby is gradually forced through the distended 

 openings of uterus and vagina and is born. This is followed by freeing of 

 the placenta from the uterine wall. With the ruptured embryonic mem- 

 branes the placenta constitutes the "afterbirth." 



Toward the close of pregnancy the mammary glands (breasts) enlarge, 

 and their glandular tissue shows a marked increase in amount. These 

 changes are produced by estrogen and progesterone, to the production 

 of which the placenta increasingly contributes. Milk production begins 

 about 3 days after the birth of the child and is caused by one of the 

 gonadotrophic hormones, prolactin (which may prove to be the same as 

 luteotrophin) . So long as estrogens are present in the blood in large 

 amounts, the secretion of prolactin is inhibited. The sudden fall in estrogen 

 level at the time of birth releases prolactin into the blood. Continued 

 secretion of this pituitary hormone is necessary for continued milk pro- 

 duction, and this is apparently brought about by nervous impulses arising 

 from sensory stimulation of the nipples in nursing the child. Soon after 

 the infant is weaned, milk production ceases. 



