ORIGIN OF PLACENTAL MAMMALS, 247 
of a decidua, but also in the external form of the placenta 
itself. In the Indeciduata it consists, in most cases, of 
numerous, single, scattered bunches or tufts of vessels, and 
hence this group may be called tufted placental animals, 
(Villiplacentalia). In the Deciduata, however, the single 
tufts of vessels are united into a cake, which appears in two 
different forms. In the one case it surrounds the embryo in 
the form of a closed band or ring, so that only the two poles 
of the oval ege bladder are free of tufts; this is the case in 
animals of prey (Carnaria) and the pseudo-hoofed animals 
(Chelophora), which may consequently be comprised as 
girdled-placental animals (Zonoplacentalia). In the other 
Deciduata, to which man also belongs, the placenta is a 
simple round disc, and we therefore call them disc-placen- 
tals (Discoplacentalia). This group includes the five orders 
of Semi-apes, Gnawing animals, Insectivora, Bats, and Apes, 
from the latter of which, in the zoological system, man 
cannot be separated. 
It may be considered as quite certain, from reasons based 
upon their comparative anatomy and their history of de- 
velopment, that Placental animals first developed out of 
Marsupials, and that this very important development—the 
first origin of the placenta—probably took place in the 
beginning of the tertiary epoch, during the eocene period. 
But one of the most difficult questions in the genealogy of 
animals is the important consideration whether all Placental 
animals have arisen out of one or out of several distinct 
branches of Marsupials; in other words, whether the origin 
of the placenta occurred but once, or several times. 
When, in my General Morphology, I for the first time 
endeavoured to establish the pedigree of Mammals, I here, 
