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 placentse are always attached to the mesometrial 

 border of the uterine cornu. 



In the Carnivora the umbiUcal 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 reflexa 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 Simiadae. 



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 Simiadse, 

 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 corre- 



