92 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



decidua forms first a nest for it, and then grows over and buries it, 

 thus forming the outer or decidual membrane of the embryo. The 

 layer which covers the vesicle (see Fig. 71) is the decidua reflexa; 

 the part by which it adheres to the uterus, the decidua serotina; 



decidua reflexa 



decidua vera 

 jdecidua serotina 



.blastodermic vesicle 

 decidua vera 



decidua vera 

 ervix 



vagina 



Fig. 71.— Section of the Uterus showing the three parts of the Decidua. 



the rest, lining the uterus, the decidua vera. As the vesicle 

 grows, it fills the cavity of the uterus, thus bringing the decidua 

 reflexa in contact with the decidua vera ; the reflexa disappears 

 when it comes in contact with the vera, which remains as the 

 outermost of the foetal membranes. The serotina takes part in 

 the formation of the placenta. 



The Amnion and Chorion. — Only part of the somatopleure 

 of the blastodermic vesicle takes part in the formation of the 

 embryo ; the remaining parts of the somatopleure are converted 

 into the amnion and chorion in this manner : 



As is shown in Fig. 70, the embryonic area appears to sink 

 within the vesicle ; the somatopleure rises up as a wave-like fold 

 right round it, at each side, at the head as well as at the tail end. 

 The folds meet over the embryo and unite, as is shown in 

 Fig. 72. Thus the embryo is wrapped up in a double fold of 

 its own somatopleure. The inner fold or membrane is the 

 Amnion, the outer is the Prechorion (Fig. 72). The prechorion, 

 in turn, is covered by the decidua reflexa, and at its attached 



