CH. LVIII.] 



THE FffiTAL MEMBRANE?. 



8I 7 



partly of maternal tissue (decidua serotina) ; partly of foetal 

 tissue (chorion). Within the chorion is another foetal mem- 

 brane, which is attached to the ventral wall of the embryo ; 

 it is called the amnion. This forms a sheath to the allantoic 

 stalk or umbilical cord, and is then reflected over the rest of the 

 embryo. In the umbilical cord are seen the remains of the yolk- 



fig. 631. Diagram of an early stage of the formation of the human placenta, a, 

 embryo ; b, amnion ; c, placental vessels ; <l, decidua reflexa ; , chorion ; /, placental 

 villi; y, mucous membrane or decidua vera. (Cadiat ) 



sac or umbilical vesicle. The amnion is filled with amniotic fluid 

 in which the foetus floats, and is thus protected from external 

 violence. The os uteri is closed by a plug of mucus. 



Fig. 631 is a rather more diagrammatic view of the same 

 structures in outline, the villi over the general surface of the 

 chorion having disappeared. 



Development of the Decidua. 



The ovum which has been fertilised in the Fallopian tube usually 

 arrives in the uterus in the condition of the trilaminar blastoderm. 

 It is larger than the undeveloped ovum, but still it is extremely 

 small. The mucous membrane (decidua) becomes thicker, more 

 pulpy, and has longer glands than it would have had if fertilisation 

 had not occurred. In this the ovum is speedily imbedded, 

 usually near the fuudus of the uterus ; the mucous membrane 

 grows over the little ovum and encloses it, and so the decidua 

 K.P. 3 G 



