118 



TEXT-BOOK OF EMBRYOLOGY. 



extension of the mucosa over the ovum or that part of the mucosa under which 

 the ovum buries itself (Fig. 107). 



It will be remembered that surrounding the entire young ovum is the chorion 

 and that this membrane consists of two layers, an outer ectoderm (trophoderm) 

 and an inner mesoderm. In the youngest known human embryo the chorion is 



Decidua parietalis 

 Decidua capsularis 



Decidua basalis ] 

 Chorion frondosum I 



Placenta 



FIG. 107. Semidiagramatic sagittal section of human uterus containing an 



embryo of about five weeks. Allen Thompson. 



a, Ventral (anterior) surface; c, cervix uteri; ch, chorian; g, outer limit of decidua; 

 m, muscularis; p, dorsal (posterior) surface. 



a shaggy membrane, its entire surface being covered with small projections or 

 villi. Later these villi disappear from all of the chorion except that part of it 

 which becomes attached to the uterine mucosa and forms the foetal part of the 

 placenta. The latter is known as the chorion frondosum, while the smooth 

 remainder of the chorion is known as the chorion lave. 

 There are thus to be considered: 



1. The decidua parietalis. 



2. The decidua capsularis. 



3. The decidua basalis 



4. The chorion frondosum 



forming the placenta. 



