PREGNANCY AND PARTURITION 



1377 



proliferate, the stroma cells are enlarged, and the blood-capillaries 

 are widely dilated. The stroma cells become converted into the 

 large decidual cells. By the time the fertilised ovum arrives in the 

 uterus the process of hypertrophy and loosening of 

 the layers of the mucous membrane has already 

 made some progress. As it lies on the mucous 

 membrane, the outermost cells of the developing 

 ovum exercise a destructive influence on the ad- 

 jacent cells of the mucous membrane, apparently 

 through some sort of digestion, so that the ovum 

 sinks in the membrane and reaches the subepithelial 

 connective tissue. Round the margins of the de- 

 pression, which the ovum has made for itself, the 

 mucous membrane grows over the protruding part of 

 the ovum (Fig. 562). When this has taken place, 

 the different parts of the mucous membrane receive 

 different names. Since (in man) they are all to be 

 cast off with the foetus at birth, each part is spoken 

 of as the decidua, that lining the main body of the 

 uterus being known as the decidua vera, that cover- 

 ing the protruding part of the egg as the decidua 

 reflexa, while that to which the egg is immediately 

 attached is the decidua serotina or basalis. . It is from 

 the latter that the placenta is formed. By the end 

 of the second week the blood-vessels in this situa- 

 tion are considerably enlarged. This enlargement 

 proceeds/ affecting especially the capillaries and 

 veins, until these form venous sinuses at the junc- 

 tion between the mucous membrane and the 

 muscular coat. Changes take place at the same 

 time in the embryo. When it sinks into the mucous 

 membrane it has a diameter of 1 mm. The blasto- 

 derm is fully formed with its three layers ; the yolk- 

 sac, the body cavity, and the amnion are present. 

 The outermost layer of the epiblast becomes 

 specially modified to serve for the nutrition of , the pregnancy" 

 embryo, and gives rise to the production of 

 numerous villi, the chorionic villi, so that the 

 whole ovum has a shaggy appearance. Since this tissue takes no 

 part in the further development of the embryo, but serves simply 

 for its nutrition, it is often spoken of as the trophoblast. With the 

 formation of foetal blood-vessels, these penetrate into the villi, together 

 with mesoblast. The villi grow into the venous spaces, especially in 

 the basal part of the decidua, so that, at this period, the foetal 



87 



'm. 561. Isolated 

 muscle- cells from 

 the uterus, showing 

 the hypertrophy 

 during pregnancy. 

 a, fibre from uterus 

 in ninth month of 



