FORMATION OF THE PLACENTA. 



811 



chorion. Sometimes, at full term, the membranes of the foetus can be sepa- 

 rated from the decidua ; but frequently all of the different layers are closely 

 adherent to each other. 



The changes just described are not participated in by the mucous mem- 

 brane of the neck of the uterus. The glands in this situation secrete a semi- 

 solid, transparent, viscid mucus, which closes the os and is sometimes called 

 the uterine plug. 



Toward the fourth month a very delicate, soft, homogeneous layer ap- 

 pears over the muscular fibres of the uterus, beneath the decidua vera, which 

 is the beginning of a new mucous membrane. This is developed very gradu- 

 ally, and the membrane is completely restored about two months after partu- 

 rition. 



Formation of the Placenta. At about the end of the second month the 

 villi of the chorion become enlarged and arborescent over that part which 

 eventually forms the foetal portion of the placenta. They are then highly 

 vascular and are embedded in the soft substance of the hypertrophied mucous 



FIG. 297 .Diagrammatic figure, showing the placenta and deciduce (Liegeois). 



c, embryon ; f, intestine ; p, pedicle of the umbilical vesicle ; o, umbilical vesicle ; TO, m, m, amnion ; 

 a', chorion ; a, lower end of the umbilical cord ; q. q, vascular tufts of the chorion, constituting the 

 foetal portion of the placenta ; n', n, maternal portion of the placenta ; n, n, decidua vera ; s, decid- 

 ua reflexa. 



membrane. At the same time the villi over the rest of the chorion are ar- 

 rested in their growth, and they finally disappear during the third month. 

 The blood-vessels penetrate the villi in the form of loops at about the fourth 

 week ; and the placenta is distinctly marked at about the end of the third 



