FORMATION OF THE PLACENTA 813 



sanguinolent liquid ; but this disappears at about the end of the fourth 

 month, and the membranes then come in contact with each other. They 

 soon become so closely adherent as to form a single membrane, which 

 is in contact with the chorion. Sometimes, at full term, the membranes 

 of the foetus can be separated from the decidua ; but frequently the dif- 

 ferent layers are closely adherent to each other. 



The changes just described are not participated in by the mucous 

 membrane of the neck of the uterus. The glands in this situation 

 secrete a semisolid, transparent, viscid mucus, which closes the os and 

 is sometimes called the uterine plug. 



Toward the fourth month a very delicate, soft, homogeneous layer 

 appears over the muscular fibres of the uterus beneath the decidua vera, 

 which is the beginning of a new mucous membrane. This is developed 

 very gradually, and the membrane is completely restored about two 

 months after parturition. 



Formation of the Placenta. At about the end of the second month 

 the villi of the chorion become enlarged and arborescent over that part 

 which eventually forms the foetal portion of the placenta. They are 

 then highly vascular and are embedded in the soft substance of the 

 hypertrophied mucous membrane. This portion of the chorion is called 

 the chorion frondosum. At the same time the villi over the rest of the 

 chorion (chorion leve) are arrested in their growth, and they finally dis- 

 appear during the third month. The bloodvessels penetrate the villi in 

 the form of loops at about the fourth week; and the placenta is dis- 

 tinctly marked at about the end of the third month. The placenta then 

 rapidly assumes the anatomical characters observed after it may be said 

 to be fully developed (see Plate X, Fig. 6). 



The fully-formed placenta occupies about one-third of the uterine 

 mucous membrane, and usually is rounded or ovoid in form, with a dis- 

 tinct border connected with the decidua and the chorion. It is seven to 

 nine inches (18 to 23 centimeters) in diameter, a little more than an inch 

 (2.5 centimeters) in thickness at the point of penetration of the um- 

 bilical cord, slightly attenuated toward the border, and weighs fifteen 

 to thirty ounces (425 to 850 grams). Its foetal surface is covered with 

 the smooth amniotic membrane, and its uterine surface, when detached, 

 is rough, and divided into irregular lobes, or cotyledons, half an inch to 

 an inch and a half (12.7 to 38.1 millimeters) in diameter. Between these 

 lobes are membranes, called dissepiments, which penetrate into the 

 substance of the placenta and at its border extend as far as the foetal 

 surface. 



On the uterine surface of the placenta, is a thin soft membrane, the 

 decidua serotina. This is composed of amorphous matter, a large 



