60 



EMBRYOLOGY 



Mucous membrane 



Muscular fibers 



Stratum compactum 



the free surface; in this the uterine glands are only slightly expanded, and are 

 lined by columnar cells; (2) stratum spongiosum, in which the gland tubes are greatly 

 dilated and very tortuous, and are ultimately separated from one another by only 

 a small amount of interglandular tissue, while their lining cells are flattened or 

 cubical; (3) a thin unaltered or boundary layer, next the uterine muscular fibers, 

 containing the deepest parts of the uterine glands, which are not dilated, and 



are lined with columnar epithelium; 

 it is from this epithelium that the 

 epithelial lining of the uterus is re- 

 generated after pregnancy. Distinc- 

 tive names are applied to different 

 portions of the decidua. The part 

 which covers in the ovum is named the 

 decidua capsularis; the portion which 

 intervenes between the ovum and the 

 uterine wall is named the decidua 

 basalis or decidua placentalis ; it is here 

 that the placenta is subsequently 

 developed. The part of the decidua 

 which lines the remainder of the body 

 of the uterus is known as the decidua 

 vera or decidua parietalis. 



Coincidently with the growth of 

 the embryo, the decidua capsularis is 

 thinned and extended (Fig. 34) and 

 the space between it and the decidua 

 vera is gradually obliterated, so that 

 by the third month of pregnancy the 

 two are in contact. By the fifth 

 month of pregnancy the decidua cap- 

 sularis has practically disappeared, 

 while during the succeeding months 

 the decidua vera also undergoes 

 atrophy, owing to the increased press- 

 ure. The glands of the stratum com- 

 pactum are obliterated, and their 

 epithelium is lost. In the stratum 

 spongiosum the glands are compressed 

 and appear as slit-like fissures, while 

 their epithelium undergoes degener- 

 ation. In the unaltered or boundary 

 layer, however, the glandular epithe- 

 lium retains a columnar or cubical 

 form. 



Stratum spongiosum 



Unaltered or 

 boundary layer 



Muscular fibers 



FIG. 33. Diagrammatic sections of the uterine mucous 

 membrane: A. The non-pregnant uterus. B. The preg- 

 nant uterus, showing the thickened mucous membrane 

 and the altered condition of the uterine glands. (Kundrat 

 and Engelmann.) 



The Chorion (Figs. 23 to 28) The 

 chorion consists of two layers : an outer 

 formed by the primitive ectoderm or 

 trophoblast, and an inner by the soma- 

 tic mesoderm; with this latter the amnion is in contact. The trophoblast is made 

 up of an internal layer of cubical or prismatic cells, the cytotrophoblast or layer 

 of Langhans, and an external layer of richly nucleated protoplasm devoid of cell 

 boundaries, the syncytiotrophoblast. It undergoes rapid proliferation and forms 

 numerous processes, the chorionic villi, which invade and destroy the uterine 

 decidua and at the same time absorb from it nutritive materials for the growth 



