FCETAL MEMBRANE. 191 



external or "false" amnion (composed of epiblast and somatopleure) 

 and over a small area from the allantois besides the vitelline mem- 

 brane of the ovum, which early disappears. The chorion exhibits 

 externally (toward the uterine wall) an epiblastic epithelial layer, de- 

 rived from the external amnion ; and internally a connective-tissue 

 layer of mesoblastic origin, derived from the amnion and allantois. 

 The chorion is connected with the foetus by a vascular mesoblastic stalk 

 (the umbilical cord) made up mainly of the allantois. 



The allantois is a saccular outgrowth from the primitive intestine, 

 extending to the chorion and uniting with the latter over a greater or 

 less area. It consists of mesoblast (splanchnopleure) externally and 

 internally has a cavity lined with hypoblast ; in man the portion of 

 the allantoic cavity beyond the abdominal wall consists only of a small 

 tube extending into the umbilical cord. The umbilical bloodvessels 

 are derived from the allantois. The mesoblastic tissues of the allantois 

 enter into the formation of the chorion and umbilical cord. 



At a very early period numerous vascular tufts, chorial villi, grow 

 out from the chorion on all sides, projecting toward the uterine wall. 

 A portion of the chorion, called the " chorion frondosum," enters into 

 the formation of the placenta, the villi of this region undergoing great 

 development and becoming intimately united with the corresponding 

 portion of the decidua (the decidua serotina). Over the remainder of 

 the choriou, the " chorion laeve," the villi disappear, this portion of 

 the chorion becoming united with the decidua vera to form (with the 

 amnion) the foetal membranes. 



The decidua, already described, is the modified uterine mucosa in 

 three divisions the serotina, vera, and reflexa. 



The umbilical vesicle, or " yolk sac," in the higher mammals has 

 little functional value, and soon disappears. It consists of a sac or 

 vesicle situated between the amnion and chorion, and a pedicle or 

 stalk in the umbilical cord. In origin it is continuous with the primi- 

 tive intestine, and consists of a vascular mesoblastic (splanchnopleural) 

 layer externally, and is lined with hypoblast internally. 



The foetal membrane (" bag of waters ") is a thin, translucent mem- 

 brane given off from the margin of the placenta, forms a closed sac 

 about the foetus, and is closely applied to the inner surface of the 

 uterus. It is filled with a quantity of serous fluid, the liquor amnii, 

 in which the foetus is immersed. It is formed, from within outward, 

 of the amnion, chorion, and decidua vera. 



The amnion is a thin, non-vascular membrane, lined on its free sur- 

 face (that directed toward the foetus) by simple squamous epithelium, 

 beneath which is a connective-tissue stratum composed of connective- 

 tissue cells (chiefly flat, stellate or irregular, many with anastomosing 

 processes), fibres, and gelatinous intercellular substance. In places the 

 outer surface exhibits small hexagonal endothelioid cells. The amnion 

 is loosely connected with the underlying chorion by fibrous strands, 

 or very delicate muco-fibrous tissue, so that it has some play over the 

 latter and can be stripped off. 



The liquor amnii, which surrounds the foetus and fills the amniotic 

 sac, is a serous fluid holding proteid and other substances in solution. 



