FLACEXTA. 377 



the membrane is completely restored about two months after 

 parturition. 



Such are the changes in the mucous membrane of the 

 uterus which result in the formation of the membranae deci- 

 dua?. We shall not discuss the old ideas of the formation of 

 a decidua by an exudation from the mucous membrane, or 

 the view that the mucous membrane extends over the open- 

 ings of the Fallopian tubes, and that a portion of it is pushed 

 into the uterus by the descending ovum to form the decidua 

 reflexa, as these considerations have now only an historical 

 interest. The mode of development of the decidua reflexa 

 by a growth of the mucous membrane of the uterus over the 

 ovum was discovered by Coste ; * and this view has been but 

 little modified by the observations of Reichert, which we 

 have quoted so fully. 



Development and Structure of the Placenta. In describ- 

 ing the formation of the membranse deciduse and of the 

 chorion, we have necessarily hinted at the mode of develop- 

 ment of the placenta. Although there is considerable differ- 

 ence of opinion among anatomists with regard to the exact re- 

 lations between the vessels of the mother and of the foetus in 

 uterOj it is admitted by all that the foetus derives its nourish- 

 ment from the maternal blood, and that the placenta is, in ad- 

 dition, a respiratory organ. Reasoning from what we should 

 consider to be the requirements of the foetus, it would be 

 natural to suppose that the foetal vessels are bathed in mater- 

 nal blood ; and it is certain that the two sets of vessels have 

 no direct communication with each other. It is also well 

 known that the foetus has an independent circulation, its heart 

 beating about twice as fast as the heart of the mother. In 

 our description of the placenta, we shall first give the views 

 which we conceive to be correct, and then advance the facts 

 and arguments by which these views are apparently supported. 



1 COSTE, Developpemcnt des corps organises, Paris, 1849, Explanations of Plates 

 ii., ii*., vii., espece humaine. 



