70 



AMNION 



connective-tissue cells by a process of enlargement, and also by multiplication of the 

 elements. The decidua is also invaded by foetal tissue, as will be explained later. 



Before proceeding to the description of the development of the placenta, it 

 will be convenient to consider first the later history of the amnion and chorion. 



Amnion. As we have seen in an earlier section, there is good reason for 

 believing that in man, apes, and monkeys the amnion is closed from the beginning. 



FlG. 97. DlAGBAMMATIC SECTION OF THE PBEGNANT HUMAN UTERUS AT THE SEVENTH OB 



EIGHTH WEEK. (Allen Thomson.) 



c, c, openings of Fallopian tubes into the cavity of the uterus; c', cervix, filled by a plug of mucus: 

 the letters c and c 1 are placed within the original cavity of the uterus; dv, decidua vera; dr, decidua 

 capsularis; ds, decidua basalis ; ch, chorion with its villi growing into the decidua capsularis and 

 d. serotina: in the former the villi are becoming atrophied (chorion Iseve) ; 11, umbilical cord, the dotted 

 lines indicate blood-vessels within it ; al, allantois ; ?/, yolk-sac (umbilical vesicle) ; y 1 , its stalk, passing 

 in the umbilical cord and connected with the intestine of the embryo, i ; am, amnion. 



In all the earliest known normal embryos it forms a thin membrane over th< 

 embryonic shield, consisting of an inner layer of flattened ectoderm-cells and an outer 

 layer of mesoderm. At first, as will be noticed in a later section, it closely invests 

 the embryo, but at the beginning of the second month it is distended into a sac oi 

 considerable dimensions containing an albuminous fluid the liquor amnii in whicl 



