THE PLACENTA OF THE EAT. 93 



by the Pig may be termed, with Von 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-deciduate."* But, whatever terms be 

 employed, the question for the classifier is to inquire what mam- 

 mals correspond with Man and what with the Pig, and whether 

 the groups of deciduate and non-dedduate Nonodelphia 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 Deciduata, it is certain that the Apes 

 agree, in the main, with man in placenta!, 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 placentae differ from those of Man only in presenting a more 

 marked lobation a character which occurs as a variety in Man. 



The Cheiroptera, Insectivora, and Eodentia agree with Man 

 in possessing a placenta which is not only as much " discoidal," 

 allowance being made for the shorter curve of the uterine walls, 

 as his, but also entirely resembles his in being developed in 

 conjunction with a decidua. This decidua always corresponds 

 to at least the decidua serotina of Man ; frequently there is a 

 well-developed decidua reflexa.^ How far a decidua vera can be 

 said to be developed is doubtful. 



Figure 43 represents a section of the uterus, chorion, and 

 partially-injected placenta of a Hat (the foetus being one inch and 

 a quarter long), taken in a direction perpendicular to the long axis 

 of the uterine cornu. a is the mesometrium traversed by a 

 large uterine vein ; & is the wall of the uterus becoming looser in 



* 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. 



t See upon this subject the recently-published valuable essay of Reichert : 

 " Beitr'age zur Entwickelungs-geschichte des Meerschweinchens." Reichert finds 

 a complete, or almost complete, decidua reflexa in Rats, Mice, Guinea-pigs, and 

 Bats ; while in Rabbits, Hares, and Carnivora, the decidua reflexa only partially 

 surrounds the ovum. 



