RELATIONS OF THE EMBRYO TO THE UTERUS. 



125 



aternal tissue, r,r, which rises from the walls of the uterus and completely encap- 

 sules the ovum. The arrangement of the parts is illustrated in figure 70, 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 attached on the dorsal side of the uterus. This is the 

 normal position. The diagrams so com- 

 monly met with which represent the in- 

 sertion 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 trophoderm has now quite 

 disappeared, except so far as it persists to 

 cover the villi. The villi themselves are 

 shaggy and more or less branched. 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 amnion. 



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 71. The mucosa is enormously hyper- 

 trophied and contains a great many dilated 

 irregular blood-sinuses. From the dorsal 

 side of the organ is suspended a large ', Anterior, & posterior surface, g, Outer limit of 



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



FIG. 70, SEMI-DIAGRAMMATIC OUTLINE OF AN 

 ANTERO-POSTERIOR SECTION OF A HUMAN 

 UTERUS CONTAINING AN EMBRYO OF ABOUT 

 FIVE WEEKS. 



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. 

 c, Cervical canal. (After Allen Thompson.) 



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 villous chorion of the ovum, and find, as above stated, 

 that only the tips of the villi are united with the surface of the reflexa. In the 

 fresh state the decidua is reddish gray, spongy or pulpy, soft, and moist. After the 

 fourth month it acquires, especially in the superficial layers, a duller brownish color, 

 which subsequently becomes more marked. This coloration is due to the decidual 

 cells. During the first two or three months the scattered openings of the uterine 

 glands can still be distinguished over the surface of the serotina and vera. The 

 surfaces themselves of the vera and reflexa, though somewhat irregular, remain 



