426 THE FEMALE EEPEODUCTIYE ORGANS 



and chorion, on the maternal side by the tissues of the decidua 

 serotina, while between these boundaries is a broad interval which 

 is occupied by a forest of arborizing processes of the fetal chorion, 

 some of which, the main stems, completely span the interval, their 

 tips being firmly embedded in the surface of the decidua, while 

 others, the free branches m floating villi, pass from the lateral sur- 

 faces of the main stems and repeatedly subdivide, the tips of their 

 branches floating free in maternal blood spaces which are therefore 

 known as intervillous spaces. 



Where the fetal chorion and maternal decidua are in direct 

 contact at the margin of the placenta, the decidual cells extend 

 inward to meet the opposed surface of the chorion, upon which 

 they then expand to form the so-called decidua subcliorialis (Fig. 

 349, page 432). 



THE AMNION. This is a thin membrane, of fetal origin, con- 

 sisting of an epithelial coat and a thin layer of mesenchymal con- 

 nective tissue. Its epithelium, whose surface is directed toward' 

 the fetus, consists of cuboidal or flattened epithelial cells which 

 are derived from the ectoderm ; they are firmly united with one 

 another by means of intercellular bridges (Minot*). The connect- 

 ive tissue forms a thin transparent layer of embryonic tissue in 

 which are many cells. From the inner surface of this layer deli- 

 cate processes pass to the surface of the chorion, to which mem- 

 brane the amnion is loosely attached. 



THE CHORION. This tissue includes a membranous portion 

 which is in relation with the amnion, and a villous portion which 

 forms the forest of placental villi already described as lying 

 between the fetal membranes, on the one hand, and the maternal 

 decidua on the other. That portion of the chorion which enters 

 into the formation of the placenta is known as the chorion f rondo- 

 sum, in contradistinction to the remaining portion of the chorionic 

 membrane which is loosely attached to the decidua vera and is 

 called the chorion Iceve. 



The membranous portion of the chorion may be said to consist 

 of two layers, an inner or fetal layer of embryonic connective 

 tissue, which is continuous with the similar tissue of the amnion 

 and serves for the transmission of the fetal blood vessels on their 

 way from the umbilical cord to the placental villi, and an outer 

 layer which consists of intermingled groups of large cells, collect- 



* Loc. cit. 



