372 HISTOLOGY 



so that in the last half of pregnancy the chorion laeve rests directly against 

 the decidua vera (Fig. 371, B). 



The placenta at birth is a discoid mass of spongy vascular tissue, about 

 7 in. in diameter and i in. thick, weighing a pound. It is composed of two 

 parts, the placenta uterina and placenta fetalis, which in certain lower 

 mammals can be readily separated, but in others, and in man, they cannot, 

 be disengaged. The uterine portion, as already stated, is the decidua 

 basalis, and the embryonic or fetal portion is the chorion frondosum. At 

 the margin of the placenta, the chorion frondosum is continuous with the 

 chorion laeve, which is adherent to the decidua vera. Lining the chorionic 

 cavity and spreading from the distal end of the umbilical cord, the amnion 

 forms a complete sac, with a smooth and glistening surface toward the 

 embryo. It is lightly adherent to the chorion laeve and to that surface of 

 the placenta which is toward the embryo. From the way in which the 

 chorion Iseve and chorion frondosum become differentiated, the fact that 

 small accessory placentas sometimes occur near the main mass may be 

 readily understood; detached groups of chorionic villi continue their growth, 

 and their vessels communicate with those of the adjacent placenta. Such 

 small accessory structures are known as succenturiate (i.e., recruited) 

 placentas. 



Fate of the Membranes at Birth. Shortly before birth, the cervix of 

 the uterus dilates and the sac of membranes containing the liquor amnii 

 bulges into it. The membranes thus exposed are ruptured, and the 

 amniotic fluid escapes. The birth of the child follows, and the umbilical 

 cord then extends from the navel through the vagina to the placenta. The 

 cord is so short in some mammals that it ruptures with the expulsion of 

 the embryo; in other forms it is bitten off or otherwise severed, setting free 

 the embryo. Occasionally the membranes rupture in such a way that the 

 head of the infant remains more or less covered with a cap of amnion and 

 chorion laeve, formerly known as the "caul." After the birth of the child 

 the uterine musculature contracts quite rapidly, and in about half an hour 

 the after-birth is expelled, the sac of membranes being turned inside out 

 in this process. The part from the fundus of the uterus is forced out first, 

 and that from the lower segment of the uterus follows. Thus the amnion 

 and the amniotic surface of the placenta are on the outside of the after- 

 birth. The denuded uterine mucosa is gradually restored to its normal 

 condition. As after menstruation, the epithelium spreads from the glands 

 over the tunica propria. 



The entire after-birth, since its delivery follows that of the child, was called the 

 secunda or secundina by the ancient anatomists. The round flat mass which is its 

 principal part was named the placenta by Fallopius, from its fancied resemblance to 

 a pan-cake. Long before this, the membranes enveloping the embryo were known as 

 the chorion, allantois. and amnion, and were described as the outer, middle and inner 



