RELATIONS OF THE EMBRYO TO THE UTERUS. 



119 



decidua serotina, the area (Fig. 55, s,s) to whicfr the ovum is attached; second, the 

 decidua -vera, comprising all the remaining portions of the mucosa forming part 

 of the walls of the body of the uterus ; third, the decidua reftexa, the arching dome 

 of maternal tissue, r,r, which rises from the walls of the uterus and completely 

 encapsules the ovum. The arrangement of the parts is illustrated in figure 55, 

 which represents a median section of a 

 uterus about five weeks pregnant. The 

 whole uterus is considerably enlarged. 

 The mucous lining of the uterus is very 

 greatly thickened. The ovum is at- 

 tached on the dorsal side of the uterus. 

 This is the normal position. The dia- 

 grams so commonly met with which rep- 

 resent the insertion of the ovum at other 

 points should not be accepted by the 

 student. The reflexa rises around the 

 ovum, completely covering it in so as 

 to make a closed bag. The ovum itself 

 is a sac known as the chorionic vesicle. 

 The trophoblast has now quite disap- 

 peared, except so far as it persists to 

 cover the villi. The villi themselves are 

 shaggy, more or less branched, and their 

 tips are united either with the surface of 

 the decidua serotina or with that of the 

 decidua reflexa. In the interior of the 

 chorion is lodged the embryo with its 

 yolk-sac and surrounded by the am- 

 nion. 



If the walls of the uterus are cut 

 through and simply reflected, leaving the 

 bag of the decidua reflexa intact, the ap- 

 pearances will be found essentially as in 

 figure 56. The mucosa is enormously 

 hypertrophied and contains a great many 



dilated irregular blood-sinuses. From the dorsal side of the organ is suspended 

 a large closed bag or sac, the decidua reflexa, D. ref, nearly filling the cavity of the 

 uterus. The reflexa presents in the stage figured the same general appearance as 

 the surface of the uterus. If the reflexa be open, we come, of course, upon the 



FIG. 55. SEMI-DIAGRAMMATIC OUTLINE OF AN 

 ANTERO-POSTERIOR SECTION OF A HUMAN 

 UTERUS CONTAINING AN EMBRYO OF ABOUT 

 FIVE WEEKS. 



a, Anterior, /, posterior surface, g, Outer limit of 

 the decidua s,s, Limits of the decidua serotina. 

 ch, Chorion, within which is the embryo enclosed 

 by the amnion, and attached to the chorion by 

 the umbilical cord ; from the cord hangs the 

 pedunculate yolk-sac. r,r, Decidua reflexa. 

 (After Allen Thompson.') 



