120 TEXT-BOOK OF EMBRYOLOGY. 



By about the fifth month the rapidly growing embryo with its membranes 

 has filled the uterine cavity, and the decidua capsularis, now a very thin trans- 

 parent membrane, is everywhere pressed against the decidua parietalis. It 

 ultimately either disappears (Minot) or blends with the decidua parietalis 

 (Leopold, Bonnet). 



The Decidua Basalis. As the decidua basalis is that part of the mucosa 

 to which the chorion frondosum is attached, it is convenient to consider the 

 two structures together. 



Decidua 



"Fastening" villi 



Terminal villi 



Vein 

 Chorion 



FIG. 108. Isolated villi from chorion frondosum of a human embryo of 

 eight weeks. Kollmann's Atlas. 



At a very early stage, villi develop over the entire surface of the chorion 

 (Fig. 106). Very soon, however, the villi begin to increase in number and in 

 size over the region of the attachment of the ovum and to disappear from the 

 remainder of the chorion, thus leading to the already mentioned distinction 

 between the chorion frondosum and the chorion laeve (p. 118). 



THE CHORION FRONDOSUM or fcetal portion of the placenta consists of two 

 layers which are not, however, sharply separated. 



1. The compact layer. This lies next to the amnion and consists of con- 

 nective tissue. At first the latter is of the more cellular embryonal type. Later 

 it resembles adult fibrous tissue. 



2. The villous layer. The chorionic villi, when they first appear, are short 



