PREGNANCY AND PARTURITION 



1283 



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 adjacent cells of the mucous mem- 

 brane, apparently through some sort of digestion, so 

 that the ovum sinks in the membrane and reaches the 

 sub-epithelial connective tissue. Round the margins 

 of the depression which the ovum has made for itself, 

 the mucous membrane grows over the protruding 

 part of the ovum (Fig. 577). 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 

 covering the protruding part of the egg as the decidua 

 refleoca, 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 blastoderm 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 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 tropJio- 

 blast. 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 villi are 

 immersed in maternal blood, the foetal blood vessels being separated from 

 the maternal blood by a double layer of epithelium, one layer of which is 

 maternal and the other foetal in origin. Later these cells become reduced 

 to a single layer. 



NUTRITION OF THE EMBRYO. At the earliest period of its develop- 

 ment the fertilised ovum is dependent for its nourishment on the remains 

 of the cells of the discus proligerus adhering to it, or on the fluid of the 



FIG. 576. Isolated mus- 

 cle cells from the 

 uterus, showing the 

 hypertrophy during 

 pregnancjT-. 

 a, fibre from uterus 

 in ninth month of preg- 

 nancy ; &, fibre from a 

 non-gravid uterus. 

 (After BUMM.) 



