114 



DE VEL OPMENT. 



exposed to the influence of the maternal blood circulating in the decidua or 

 uterine portion of the placenta, from which it imbibes the materials of nutrition, 

 and to which it gives up effete material, the removal of which is necessary for its 

 purification. 



The Decidua. The growth of the chorion and placenta can only be understood 

 by tracing the formation of the decidua. 



The decidua (Figs. 80 G, 88) is formed from the mucous membrane of the 

 uterus. Even before the arrival of the fecundated ovum in the uterus the mucous 



membrane of the latter is vascular 

 and tumid, and when the ovum has 

 reached the uterus it becomes im- 

 bedded in the folds of the mucous 

 membrane, which grow up around it 

 and finally completely encircle it, so 

 as to cover it in entirely and exclude 

 it from the uterine cavity. Thus two 

 portions of the uterine mucous mem- 

 brane (decidua) are formed viz. that 

 which coats the muscular wall of the 

 uterus, decidua vera, and that which 

 is in contact with the ovum, decidua 

 reflexa. The decidua vera at the os 

 internum and at the openings of the 

 Fallopian tubes is continuous with 

 the lining membrane of these canals, 

 the thickening of the original mucous 

 membrane of the uterus which con- 

 verts it into decidua abruptly ceasing 

 at these points. The neck of the 

 uterus after conception is closed by 

 a plug of mucus. The decidua vera 

 is perforated by the openings formed 

 by the enlarged uterine glands, which 

 become much hypertrophied and de- 

 veloped into tortuous tubes. It con- 

 tains at a later period numerous arte- 

 ries and venous channels, continuous 

 with the uterine sinuses, and it is 

 from it that the uterine part of the 

 placenta is developed. The portion 

 of the decidua vera which takes part 

 in the formation of the placenta is 

 called the decidua serotina (Fig. 

 88, /). 



The decidual reflexa is shaggy 



on its outer aspect, but smooth within. The vessels which it contains at first 

 disappear after about the third month. About the fifth or sixth month the space 

 between the two layers of the decidua disappears, and toward the end of preg- 

 nancy the decidua reflexa is transformed into a thin yellowish membrane, which 

 constitutes the external envelope of the ovum. 



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 Dr. 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 men- 

 struation 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 



FIG. 88. Sectional plan of the gravid uterus in the 

 third and fourth month. (From Wagner.) a. Plug of 

 mucus in the neck of the uterus. 6. Fallopian tube. e. 

 The decidua vera. c 2 . The decidua vera passing into the 

 right Fallopian tube : the cavity of the uterus is almost 

 completely occupied by the ovum, e, e. Points of reflec- 

 tion of the decidua reflexa (in nature the united decidua 

 do not stop here, but pass over the whole uterine surface 

 of the placenta), g. Supposed allantois. h. Umbilical 

 vesicle, i. Amnion. fc. Chorion, covered with the decidua 

 reflexa. d. Cavity of the decidua. /. Decidua serotina, 

 or placental decidua. 



