THE CHORION AND THE DECIDUA. 605 



run together to form a dense stratum, in whicli the nuclei remain 

 visible for a time ; ultimately the nuclei disappear, and the 

 whole layer becomes modified into a hyaline, very refractive 

 substance, permeated by numerous channels, so as to present 

 a reticular appearance, and absorbing staining reagents very 

 readily. This substance, formed by degeneration of the surface 

 epithelial cells of the chorion, has been described, before its 

 epithelial origin was known, as canalised fibrin. 



Over the villi, the deeper or cellular layer of the epithelium 

 disappears in great part, persisting only in isolated patches. 

 The surface cells become converted in great part into a fibrin 

 layer, similar to that of the chorion frondosum itself 



In the chorion Iseve the epithelium retains its cellular 

 character, and no fibrin layer is formed. 



4, The Decidua. 



The decidua is the mucous membrane of the pregnant 

 uterus. The early stages in its formation are, so far as they 

 are known, identical with those by which the catamenial or 

 menstrual decidua is formed. The mucous membrane becomes 

 thicker and more pulpy than in its quiescent condition ; the 

 blood-vessels enlarge ; the glands elongate, and their deeper ends 

 become tortuous and dilated ; the deeper part of the mucosa 

 becomes crowded with modified, and apparently proliferating 

 connective-tissue cells ; and the surface epithelium, lining the 

 uterus, together with the immediately underlying connective 

 tissue, show a tendency to disintegrate. 



Up to this point, the formation of the catamenial decidua and 

 of the decidua of pregnancy appear to be identical; the sole 

 difference between the two is that, in the former, the processes 

 having reached a certain point, stop and then become retro- 

 gressive, the decidua being broken up and discharged, together 

 with a certain amount of blood, as the menstrual fluid ; while, 

 on the other hand, in the case of the decidua of pregnancy, 

 development, after reaching the point mentioned, does not stop, 

 but proceeds to further stages of elaboration. 

 ' The difference between the two courses seems to depend 

 solely on the presence of a fertilised ovum within the uterus in 

 the latter case, and on the absence of such an ovum in the 

 former; so that the catamenial decidua may be viewed as a 



