PLACENTAL STRUCTURES OF THE TENREC. 305 
In the Rodent placenta the ramifications of the umbilical vessels have the same rela- 
tions as in the Insectivora ; but the chorion is supplied with blood-vessels from the 
omphalo-mesenteric system in its non-placental parts, which consequently take a share 
in the nourishment and respiration of the foetus. The decidua reflexa is, when not 
rudimentary from the beginning, fragmentary at the end of gestation. The deciduous 
serotina is always distinct from the placenta, and separable from it (even when not 
separated) at parturition. The placente are always attached to the mesometrial 
border of the uterine cornu. 
In the Carnivora the umbilical ramifications spread over the entire chorion, not merely 
in its placental zone, absorbing nutriment consequently from, and interchanging pro- 
ducts with, the secretions of the non-placental uterine mucous membrane. The allantois 
is here always a perfect sac, separating the exterior of the amnios from the interior 
surface of the chorion. They have no decidua refléxa properly so called. The omphalo- 
mesenteric vessels are to be found persistent within the cavity of the abdomen, as in 
the Rodents, for some time after birth ; but they never reach the chorion, as in that class. 
There is an approximation made, by the colossal maternal capillaries of certain of this 
class, to the sinus-system of the Simiade. 
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 the 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 corresponding to the four classes well 
established and acknowledged upon quite other principles of division, viz. the Simiade, 
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 Simiade; 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 corre- 
