supial 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 embry