AND THOSE OF CERTAIN OTHER MAMMALIA. 101 



In therefore the deciduate mammalia, or, in other words, in 

 those mammalia in which the foetal placenta brings away with it 

 always certain maternal elements inextricably interfused with its 

 mass, and very commonly certain other maternal elements also in 

 the form of a superposed layer of deciduous serotina, we find that, 

 by looking to the three following points — firstly, to the extent of 

 vascularity which the chorion possesses, secondly, to sources 

 whence this vascular supply comes, and, thirdly, to the relation 

 which the ultimate capillary ramifications of the umbilical vessels 

 hold to the maternal blood- vascular system — we can make four 

 classes well established and acknowledged upon quite other prin- 

 ciples of division, viz. the Simiadae, the Insectivora, the Rodentia, 

 and the Carnivora. 



The persistence of the allantois as a sac seems to be a variable 

 character within the limits of single classes ; but its relations to 

 the amnios within and to the secondary chorion without appear, 

 in this as well as in the non-deciduate division of mammals, to 

 furnish good classificatory indications. 



The development of a decidua reflexa is probably constant in 

 the Simiadae ; it is variable in extent in the Rodents and probably 

 also in the Insectivora, and it is virtually absent in the Carnivora. 



In the Rodents alone does the yelk-sac assume any physiological 

 importance, and this it does by virtue of the omphalo-mesenteric 

 vessels it carries, and at the cost of its sac-character. In the other 

 classes it is found, at the end of the period of gestation, to retain 

 this character, the structure being in a more or less atrophied 

 condition. 



The second division in Professor Weber's binary classification 

 of Mammals corresponds, if we exclude the Edentata provisionally, 

 and include, with perhaps more confidence, the Sirenia 1 , exactly 

 with the Ungulata and Mutica of Linnaeus, and with the Artio- 

 dactyla, and Perissodactyla, and Mutilata of Professor Owen's 

 classification in the Linnean Society's ' Proceedings/ Of all these 

 animals alike it may be predicated that at parturition the villi of 

 the chorion separate themselves from the maternal structures, in 

 which previously they were ensheathed, without bringing away 



1 [The diffused arrangement of the villi on the chorion of the Dugong has been 

 ascertained by Paul Harting. 'Het Ei en de Placenta van Halichore Dugong,' Utrecht, 

 1878.— Editor.] 



