230 THE FEMALE REPRODUCTIVE SYSTEM 



accepted now to be about the tenth to the thirteenth day following 

 the onset of menstruation, a follicle ruptures and discharges an 

 ovum. Upon entering the o\iduct the ovum undergoes its final 

 maturation division and is fertilized if sperm are present. 



PREGNANCY. 



Following fertilization, development begins and continues as the 

 egg passes along the oviduct toward the uterus. During this 

 passage, which takes about three days, a number of cells are formed. 

 The embryo comes to rest in some pocket or fold of the uterine 

 mucosa and continues its de\'elopment. While the uterus develops 

 to a condition of the premenstrual period, the embryo develops the 

 fetal membranes, and the chorion with tuft-like projections begins 

 to form. This extremely small globe erodes its way into the uterine 

 mucosa and, with development, establishes an intimate contact 

 with the uterus through the placenta. To understand the relation- 

 ship between the develo])ing embryo and the uterine wall, the 

 student is referred to embryological texts for developmental details 

 of the fetal membranes. 



Placenta. Among the marsupials, as in the opossum, gestation, 

 or uterine development of the young is brief and the young are 

 born relatively immature. In these cases the chorion is a smooth 

 membrane in close contact with the vascular uterine mucosa. The 

 outer face of the rudimentary yolk sac unites with the inner face 

 of the chorion, and thus food compounds of the maternal blood 

 vessels in the uterine wall are in juxtaposition with those in the 

 membranes of the embryo, and these food compounds pass into 

 the chorionic vessels to l)e transported via yolk stalk vessels to 

 the embryo. Oxygen is ol)tained similarly. Excretions })ass from 

 embryonic to maternal vessels. At the end of its uterine develo])- 

 ment the membranes surrounding the embryo i)ull loose from the 

 uterine mucosa and the young is born with little destruction of the 

 uterine mucosa. 



In higher mammals the chorion develops l)ranching vascular villi 

 that ])enetrate and erode the uterine nuicosa to establish varying 

 degrees of intimate relati()nshi])s. Such associations of eml)ryonic 

 and maternal tissues result in formation of an organ called the 

 ])lacenta where nutritive, res])irat()ry, and excretory (exchanges are 

 carried on. The uterine mucosa has in the meantime grown ovvv 

 the entire surface of the chorion. Although the ])lacenta \aries 



