788 DEVELOPMENT 



called. A third cellular layer, the mesoderm, is soon developed between 

 the other two. The fetus results from the harmonious growth of these three 

 layers, each of which is the source of certain tissues and organs. 



Important changes occur in the structure of the mucous membrane of 

 the uterus. The epithelium and subepithelial connective tissue, together 

 with the tubular glands, increase rapidly, and there is a greatly increased 

 vascularity of the whole mucous membrane, while a substance composed 

 chiefly of nucleated cells fills up the interf ollicular spaces in which the blood 

 vessels are contained. The effect of these changes is an increased thickness, 

 softness, and vascularity of the mucous membrane, the superficial part of 

 which itself forms the membrana decidua. 



The object of this increased development is the production of nutritive 

 materials for the ovum; for the cavity of the uterus shortly becomes filled 



FIG. 503. Section of the Lining Membrane of a Human Uterus at the Period of 

 Commencing Pregnancy, Showing the Arrangement and Other Peculiarities of the Glands, 

 d, d, d, with Their Orifices, a, a, a, on the Internal Surface of the Organ. Twice the 

 natural size. 



with secreted fluid, consisting almost entirely of nucleated cells in which the 

 chorion villi are embedded. 



When the ovum first enters the uterus it becomes embedded in the structure 

 of the decidua, which is yet quite soft, and in which soon afterward three 

 portions are distinguishable. These have been named the decidua vera, the 

 decidua basalis, and the decidua capsularis. 



In connection with these villous processes of the chorion there are de- 

 veloped depressions or crypts in the decidua vera, which correspond in shape 

 to the villi they are to lodge; and thus the chorionic villi become more or less 

 embedded in the maternal structures. These uterine crypts, it is important 

 to note, are not, as was once supposed, merely the open mouths of the 

 uterine follicles. 



The Placenta. During these changes the deeper part of the mucous 

 membrane of the uterus, at and near the region where the placenta is placed, 

 becomes hollowed out by sinuses, or cavernous spaces, which communicate 

 on the one hand with arteries and on the other with veins of the uterus. Into 



