CHANGES FOLLOWING IMPREGNATION 



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tubes, and segmentation of the fertilized ovum occurs on its passage to 

 the uterus. 



The passage of the ovum from the ovary to the uterus occupies probably 

 eight or ten days in the human. 



The peripheral cells, which are formed first, arrange themselves at the sur- 

 face of the yolk into membrane, the ectoderm. The deeper cells of the in- 

 terior pass gradually toward the surface, thus increasing the thickness of the 

 membrane already formed by a second, or entoderm, layer of cells, while the 

 central part of the yolk, the blastoderm cavity, remains filled only with a clear 

 fluid. By this means the yolk is shortly converted into a kind of secondary 

 vesicle, the walls of which are composed externally of the original vitelline 

 membrane, and within by the newly formed cellular layer, the blastoderm or 

 germinal membrane, as it is called. 



FIG. 501. 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. 



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 

 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- 



