FORMATION OF MEMBRANES. 



1163 



membrane. 



r jUiri'.i -ffEt 



Stratum compactum.- < 



Stratum spongiosiun. - 



an inch, and consists of the following strata : (1) Stratum compactum, next the 

 free surface, in which the uterine glands 

 are little altered and where they preserve 

 a comparatively narrow lumen lined by 

 columnar epithelium ; between the glands 

 are large numbers of decidual cells. (2) 

 Stratum spongiosum, in which the gland 

 tubes are very tortuous and greatly di- 

 lated, while their lining cells are flattened 

 or cubical. (3) Basal layer, next the 

 uterine muscular wall, in which the glands 

 are not dilated and where they retain 

 their columnar epithelium. It is through 

 this basal layer that the placenta is sepa- 

 rated after the birth of the child (Fig. 

 707). 



The decidua reflexa is gradually ex- 

 panded by the growing ovum, and ulti- 

 mately comes into contact and blends 

 with the 'decidua vera so as completely to 

 obliterate the uterine cavity. This oblit- 

 eration is followed by the degeneration of 

 the deciduae ; the glands of the decidua 

 reflexa become atrophied, and the entire 

 decidua practically disappears, while the 

 decidua vera is much thinned and its 

 glands also disappear, except their deepest 

 portions, which persist in the basal layer. 



In this manner the embryo becomes 

 surrounded by three membranes : (1) the 

 amnion, derived, in the case of reptiles, 

 birds, and some mammals, from the outer 

 layer of the mesoblast and the epiblast ; 

 (2) the chorion, formed by the allantois 

 (which is derived from the hypoblast and 

 inner layer of the mesoblast) and the false 

 amnion ; and (3) the decidua, derived 

 from the mucous membrane of the uterus. 



Much additional interest has been 

 given to the physiology of the decidua 

 by the fact, which seems to be now established by the researches of Sir John 

 Williams, that every discharge of an ovum, whether impregnated or not, is, as a 

 rule, accompanied by the formation of a decidua, and that the essence of menstrua- 

 tion consists in the separation of a decidual layer of the mucous membrane from 

 the uterus ; while in the case of pregnancy there is no exfoliation of the mem- 

 brane, but, on the contrary, it undergoes further development in the manner 

 described above. 



Formation of the Placenta. The placenta is developed partly from maternal 

 and partly from foetal tissues the maternal portion being derived from the decidua 

 serotina, the foetal from the villi of the chorion frondosum. These villi penetrate 

 the decidua serotina, which then undergoes a series of complicated and, as yet, 

 imperfectly understood changes. Decidual cells accumulate between the uterine 

 glands, while the glands with their epithelial lining undergo degeneration a 

 degeneration which does not, however, extend as deep as the basal layer, where 

 the glands persist, and retain their epithelial lining throughout the entire period 

 of gestation. Ultimately the superficial portion of the decidual tissue disappears, 

 arid the uterine vessels become expanded to form a labyrinth of freely communi- 



Basal layer.- 



Muscular fibres 



FIG. 707. Diagrammatic sections of the uterine 

 mucous membrane : (A) of the non-pregnant uterus ; 

 (B) of the pregnant uterus, showing the thickened 

 mucous membrane and the altered condition of the 

 uterine glands. 



