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, 
that 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 Rodents. The inversion of the germinal layers in 
the Muridse and Subungulata is to be regarded as a modifica- 
tion of a certainly very simple ancestral uterine development. 
When considered from the phylogenetic standpoint all the 
organs of the Rodentia show themselves to be directly deriv- 
able from the type of the Marsupialia, and without any logical 
difficulty w T e 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 Rodents 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 phylogeny 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 that between these 
two groups and the Marsupialia with Carnivoroid dentition 
there exists a very intimate relationship, which may be con- 
firmed both anatomically and embryologically. Upon this 
subject, as upon the phylogeny of the Carnivora, I shall 
venture hereafter to report to the Academy. 
