THE rLACENTALIA A XI) 1MPLACH\ T TALIA. 



91 



closely with corresponding vascular developments of the wall of 



the uterus (and so forming a " placenta "), an interchange of con- 

 stituents takes place between the foetal and the maternal blood, 

 through the separating walls of the fcetal and maternal vessels. 

 In this manner, throughout its prolonged intra-uterine life, the 

 Monodelphian foetus is supplied with nourishment and gets rid 

 of its effete products. 



As the three groups instituted by De Blainville are capable 

 of being thus clearly differentiated one from the other, the 

 distinctions between them having been only more and more 

 clearly brought out by the subsequent progress of knowledge, I 

 can see no ground for refusing to adopt his classification, or for 

 denying him that credit to which he is fairly entitled for appre- 

 hending these distinctions. Certainly, the later proposition, to 

 divide Mammals into two great groups only, Placentcdia and 

 Implacentalia, cannot be regarded as any improvement upon 

 De Blainville's system, as it ignores the important fact that the 



Fig. 39. 



Fig. 



39. — Diagrammatic section of a human pregnant uterus, with the contained ovum 

 (Longet). n, uterus; I, oviduct; c, cervix uteri; du, decidua uteri; dr, decidua 

 reflexa; ds, decidua serotina ; ch, chorion ; am, amnion ; ai, allantois ; nb, umbilical 

 vesicle; z, villi which form the foetal part of the placenta; z' , villi over the rest of 

 the chorion, which take no part in the placental function in man. 



