grow out from the embryo while it is still within the egg and become applied to the inner wall of the porous shell for the purpose of obtaining air, got their supply of oxygen, not from the outer air, but from the blood-vessels of the maternal tissues.  When this connection (called the placenta) between embryo and mother through the egg-shell became more perfect, not only oxygen but food-material was obtained from the blood-vessels of the mother; and, in consequence, it became unnecessary for the eggs to be provided with a large supply of food-yolk.  Among existing marsupial animals, which, on the whole, represent a lower type of mammalian structure than ordinary mammals, there is more food-yolk than in ordinary mammals, and less food-yolk than in the two egg-laying mammals.  In the ordinary mammals, such as the rabbit, dog, monkey, and man, there is practically no yolk whatever deposited in the egg; the egg is of minute size, and the embryo obtains most of its food from the maternal blood.

The small egg of the mammal divides into a number of cells, which form a hollow sphere; on the upper surface of this the development of organs begins with the formation of a depression which indicates the future middle line of the animal, and is, in fact, the beginning of the nervous system.  Under this is formed a straight rod of gelatinous material, the foundation of the vertebral column, and the body of the embryo is gradually pinched off from the surface of the hollow sphere.  After tracing the details of this process, Huxley proceeded as follows: 

“The history of the development of any other vertebrate animal, lizard, snake, frog or fish, tells the same story.  There is always, to begin with, an egg, having the same essential structure as that of the dog; the yolk of that egg always undergoes division, or segmentation, as it is often called; the ultimate products of that segmentation constitute the building materials for the body of the young animal; and this is built up round a primitive groove, in the floor of which a notochord is developed.  Furthermore, there is a period in which the young of all these animals resemble one another, not merely in outward form, but in all essentials of structure, so closely, that the differences between them are inconsiderable, while in their subsequent course they diverge more and more widely from one another.  And it is a general law, that, the more closely any animals resemble one another in adult structure, the longer and the more intimately do their embryos resemble one another; so that, for example, the embryos of a snake and of a lizard remain like one another longer than do those of a snake and of a bird; and the embryos of a dog and of a cat remain like one another for a far longer period than do those of a dog and a bird; or of a dog and an opossum; or even than those of a dog and a monkey.”

This general rule, that the longer the paths of embryonic development of two animals keep identical the more