DECIDUATE AND NON-DECIDUATE PLACENTA. 103 



No " decidua " is developed ; the elevations and depressions of 

 the nn impregnated uterus simply acquire a greater size and 

 vascularity during pregnancy, and cohere closely with the 

 chorionic villi, which do not become restricted to one spot, but 

 are developed from all parts of the chorion, except its poles, 

 and remain persistent in the broad zone thus formed through- 

 out foetal life. The cohesion of the foetal and maternal 

 placenta?, however, is overcome by slight maceration or post- 

 mortem change ; and, at parturition, the foetal villi are simply 

 drawn out, like fingers from a glove, no vascular substance 

 of the mother being thrown off. 



The process by which the mucous membrane of the uterus 

 returns to its unimpregnated condition after parturition in the 

 pig has not been traced. 



The extreme cases of placentation exhibited by man and 

 by the Pig may be termed, with Yon Baer and Eschricht. 

 from the character of the maternal placenta, " caducous " and 

 " non-caducous," or, from the degree of cohesion of the two 

 placentae in parturition, " coherent ,! and "incoherent;" or, 

 what perhaps would be better still, the two Mammals may 

 be spoken of as "deciduate" and " non-cleciduate."* But, 

 whatever terms be employed, the question for the classifier 

 is to inquire what mammals correspond with Man and what 

 with the Pig, and whether the groups of deciduate and non- 

 deciduate Monodelpliia thus formed, are natural groups, or, 

 in other words, contain such orders as can be shown, on other 

 grounds, to be affined. 



With respect to the deciduate Monodelphia, it is certain that 

 the apes agree, in the main, with man in placental, as in other 

 important characters ; and, so far as has hitherto been observed 

 (though our knowledge of the placentation of the Lemurs is 

 very defective), their placenta? differ from those of Man only in 

 presenting a more marked lobation — a character which occurs 

 as a variety in Man. 



* It is, of course, by no means intended to suggest by these terms, that the 

 homologue of the decidua does not exist in the " non-deciduate " Mammals. The 

 mucous membrane of the uterus becomes hypertrophied during pregnancy in both 

 the deciduate and the non-deciduate Mammals ; but it is thrown off, and so gives 

 rise to a " decidua " only in the one of these two groups. 



