128 SUBDIVISION OP MAMMALIA INTO ORDERS. 



alive, for this happens in certain animals belonging to nearly 

 all the oviparous classes, such as Insects, Fishes, and Reptiles : 

 but that the young animal is developed by means of nutrition 

 directly supplied to it by the parent, through a system of blood- 

 vessels partly belonging to the embryo, and partly to the uterus 

 or cavity in which it is contained. In oviparous animals, the 

 development is effected at the expense of the store of nourishment 

 laid up in the egg itself; but this is very small in the Mammalia, 

 because, at a very early period of its growth, the embryo begins 

 to draw its support directly from the parent. From the surface 

 of the chorion (which is the membrane that covers the egg, re- 

 sembling that which lines the shell in Birds), a number of little 

 tufts shoot out, and insinuate themselves amongst the vessels of 

 the uterus, from which they absorb a nutritious fluid, that ulti- 

 mately finds its way to the embryo. This is the sole mode in 

 which the embryo of the non-placental Mammalia is ever con- 

 nected with the parent : but in the truly viviparous sub-class, a 

 more direct communication is subsequently effected, by a set of 

 vessels proceeding from the embryo itself, which forms, by the 

 minuteness of its subdivisions, a mass of considerable size and so- 

 lidity, known as the placenta; this is applied against the interior 

 of the uterus, and draws nourishment from its vessels, much in 

 the same way that the roots of a tree imbibe moisture from the 

 soil, or that the lacteal vessels, spread out upon the walls of the 

 intestine, take up fluid from its cavity. Thus, the non-placental 

 Mammalia stop short, as it were, at a period of development, 

 which is very early or incomplete as regards the higher subdivi- 

 sion of the class. 



117. The method of division that we shall here adopt is very 

 nearly that of Cuvier. It rests principally upon the differences 

 which the Mammalia exhibit in their mode of development, in 

 the conformation of their limbs, and in their apparatus of masti- 

 cation ; parts whose modifications always involve a number of 

 other differences, in the structure of the various parts of the body, 

 in their habits, and even in their intelligence. When we take 

 the whole of these characters into account, we are led first to 

 divide the class of the Mammalia into two sub-classes, which 



