572 VERTEBRATE LIFE AND ORGANIZATION 



Uterine, 

 muscle 



Pla.cen.ta- ' 



Chorionic 

 villi 



Ubcrine 



lining 



Umbilica.1 cord 



Uterine linind 



Chorion 



mnion 



ervix 



YolK sa.c 



Figure 28.7. A young human embryo surrounded by its extraembryonic mem- 

 branes and lying within the uterus. Notice that the whole complex of embryo and 

 membranes is embedded in the uterine lining. Villi are present all over the surface 

 of the chorion at this stage, but only those on the side toward the uterine wall enlarge 

 and contribute to the delinitive placenta. (Modified after Patten.) 



Birth. As the embryo develops, the uterus enlarges considerably 

 to accommodate it. At the time of conception, the human uterus does 

 not protrude lar above the pubic symphysis (Fig. 28.5), but nine 

 months later, when embryonic development has been completed, it 

 extends up in the abdominal cavity nearly to the level of the breasts. 

 During this enlargement, the individual muscle fibers in its wall 

 increase in size, and additional muscle develops from undifferentiated 

 cells in the uterine wall. The uterus becomes a powerful muscular 

 organ ready to assume its role in childbirth, or parturition. 



The factors that initiate birth are uncertain, but hormones pro- 

 duced by the pituitary, ovary and the placenta itself have prepared 

 the mother's body for the birth. The mammary glands have enlarged 

 and are ready for milk production, the uterine musculature has in- 

 creased, and the pubic and other pelvic ligaments have relaxed so that 

 the pelvic canal can enlarge slightly. Birth begins by a series of involun- 

 tary uterine contractions, "labor," that gradually increase in intensity 

 and push the fetus, generally head first, against the cervix. The cervix 

 gradually dilates, but in human begins as much as 18 hours or more 

 may be required to completely open the cervical canal at the first birth. 

 The sac of amniotic fluid that surrounds the fetus acts as a wedge and 

 also helps to open the cervix. The amnion normally ruptures during 

 this process, and the amniotic fluid is discharged. \Vhen the head 

 begins to move down the vagina, particularly strong uterine contrac- 

 tions set in, and the baby is born within a few minutes. A few more 

 contractions of the uterus force most of the fetal blood from the 



