294 On the Relationship of the Rodentia to the Marsupialia. 



direct line consists in the numerous and striking similarities 

 which occur in the two groups during embryonic develop- 

 ment. If the yelk-sac of the opossum during its uterine 

 existence is of considerable extent, and at the moment of 

 birth considerably exceeds the allantois in size, so also in the 

 Rodentia, e. g. rabbits and squirrels, the yelk-sac continues 

 comparatively large during the whole period of pregnancy 

 and the allantois small. In both groups the same course 

 of development may be recognized, except that by the fusion 

 of the allantochorion with the uterine mucous membrane, 

 tliat is to say by the formation of a discoidal placenta, the 

 function of the allantois is greatly increased. But the original 

 conditions of the phylogenetic history may be inferred from 

 the volume of the yelk-sac equalling that of the allantois for 

 a long time. 



A disciform vascular area with acordifugal sinus terminalis 

 upon the yelk-sac appears in perfectly homologous develop- 

 ment in Marsupials, rabbits, and squirrels. The long persis- 

 tence of an ecto-entodermal proamnion, which in the opossum 

 is retained until birth, is likewise demonstrable in the above- 

 mentioned Eodents. The inversion of the germinal layers in 

 the Murida3 and Subungulata is to be regarded as a moditica- 

 tion of a certainly A^ery simple ancestral uterine development. 



When considered from the phylogenetic standpoint all the 

 organs of the Eodentia sliow themselves to be directly deriv- 

 able from the type of the Marsupialia, and without any logical 

 difficulty we may recognize step by step in the existing forms 

 the stages which render the transformation of long-inherited 

 arrangements intelligible. This fact has not struck me alone ; 

 it has forced itself directly upon every naturalist who has 

 studied the different organs of the Kodents from the point of 

 view of comparative anatomy, and I can only lay claim to 

 the merit of having tested the correctness of the various scat- 

 tered statements and combined them into a simple theory. 



In the present report I have only expressed my views as 

 to the pliylogeny of the Rodentia without referring to other 

 Mammalia. But I would not thereby convey the impression 

 that I have occupied myself with that group alone ; on the 

 contrary, I have also taken other divisions into the range of 

 my investigations, and have been led, with regard to the 

 Insectivora and Bats, to the conclusion tliat between these 

 two groups and the Marsupialia with Carnivoroid dentition 

 there exists a very intimate relationship, which may be con- 

 firnu'd both anatomically and cnibryologically. Upon this 

 subject, as u])on the j)hylogeny of the Carnivora, I shall 

 venture hereafter to report to the Academy. 



