THE DECIDUA. 609 



that of the uterus as a whole, and in consequence the decidua 

 reflexa ultimately comes in contact with the decidua vera, and so 

 completely obliterates the cavity of the uterus (cf. Fig. 254). This 

 usually occurs about the sixth month : the two layers, decidua 

 reflexa and decidua vera, are generally described as not only 

 coming in contact, but as fusing more or less completely together, 

 so as to form a single membrane ; but according to Minot's 

 observations, the decidua reflexa, which early undergoes degene- 

 rative changes, is entirely absorbed by the sixth month, so that 

 the chorion comes into contact with the decidua vera. 



So long as the uterine cavity remains an actual one, i.e. up 

 to the time when the chorion meets with the decidua vera, there 

 remains an open passage from the vagina, through the uterus and 

 along the Fallopian tube, to the ovary; and it is, at least theo- 

 retically, possible for spermatozoa to reach the ovary, and for 

 what is termed super-foetation to occur. 



The decidua serotina is simply the part of the decidua with 

 which the impregnated ovum comes in contact, on entering the 

 uterus, and to which it adheres ; and it is at first, therefore, 

 identical in structure with the decidua vera. It very early, 

 however, acquires special characters, owing to the chorionic villi 

 of the ovum becoming intimately connected with it. For some 

 time longer, the decidua serotina and decidua reflexa still remain 

 closely similar to each other ; but towards the end of the second 

 month (Fig. 254), the chorionic villi opposite the decidua re- 

 flexa begin to dimmish in size and importance, and to show 

 signs of degenerative changes ; while those in connection with 

 the decidua serotina become much larger and more complicated. 

 The relations between the foetal villi and the maternal tissues 

 become still more intricate, and gradually the complex and 

 elaborate structure of the fully formed placenta is acquired. 



With regard to the detailed changes that take place in the 

 different parts of the decidua during pregnancy, our knowledge 

 is still imperfect in many important respects. 



In the region lined by the decidua vera, the mucous mem- 

 brane becomes greatly thickened ; and the uterine glands be- 

 come dilated and elongated, and acquire exceedingly tortuous 

 courses. By the end of the fifth month the mucous membrane 



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