810 GENERATION. 



canal extending from the bladder to the umbilicus, which is closed soon after 

 birth. 



MembrancB Deciduce. In addition to the two membranes connected with 

 the foetus, there are two membranes formed from the mucous membrane of 

 the uterus, which are derived from the mother and which serve still farther 

 to protect the ovum. The chorion is for the protection of the foetus ; but a 

 portion of this membrane about one-third of its surface becomes closely 

 united with a corresponding portion of the uterine mucous membrane, to 

 form the placenta. 



As the fecundated ovum descends into the uterus, it is invested with a 

 shaggy covering, which is either the permanent chorion or one of the mem- 

 branes which invests the ovum previous to the complete development of the 

 allantois. At this time the mucous membrane of the uterus has undergone 

 certain changes by which it is prepared for the reception of the ovum. The 

 changes which this membrane undergoes in menstruation have already been 

 described. It has been seen that during an ordinary menstrual period, the 

 membrane is increased three or four times in thickness and becomes more or 

 less rugous. If a fecundated ovum descend into the uterus, the changes in 

 the mucous membrane progress. The glands enlarge and the mucous mem- 

 brane becomes thicker, so that at the end of the first month it measures 

 about two-fifths of an inch (10 mm.). This thickening is due chiefly to de- 

 velopment of tissue between the glands, and the membrane becomes soft and 

 pulpy. In the mean time the ovum has effected a lodgement between the 

 folds, usually at the fundus, near the opening of one of the Fallopian tubes; 

 and the adjacent parts of the mucous membrane extend over the ovum so 

 that it is at last completely enclosed. This occurs at the twelfth or thirteenth 

 day (Reichert). The extension of the mucous membrane which covers the 

 ovum becomes the decidua reflexa ; the changed mucous membrane which 

 lines the uterus becomes the decidua vera ; and the portion of the mucous 

 membrane which remains at the site of the placenta becomes the decidua 

 serotina. The vascular villosities of the chorion probably do not, as was 

 once thought, penetrate the uterine tubules, but they become surrounded by 

 tissues developed between these tubules. , 



As development advances, the decidua vera becomes extended, loses its 

 vessels and glands and is reduced to the condition of a simple membrane. 

 The cylindrical epithelium of the mucous membrane of the body of the ute- 

 rus, soon after fecundation, becomes exfoliated, and its place is supplied by 

 flattened cells. This change is effected at the sixth or the eighth week. The 

 epithelium of the cervix retains its cylindrical character, but most of the 

 cells lose their cilia. The decidua reflexa, which is thinner than the decidua 

 vera, has neither blood-vessels, glands nor epithelium. 



During the first periods of utero-gestation, the two layers of decidua are 

 separated by a small quantity of an albuminous and sometimes a sanguinolent 

 fluid ; but this disappears at about the end of the fourth month, and the 

 membranes then come in contact with each other. They soon become so 

 closely adherent as to form a single membrane, which is in contact with the 



