es place in the case of the ovum of the dog,
Professor Huxley continues: "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 yelk of that egg undergoes division or
segmentation, as it is 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 larger 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
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."[219]

We thus see that the study of development affords a test of affinity in
animals that are externally very much unlike each other; and we
naturally ask how this applies to man. Is he developed in a different
way from other mammals, as we should certainly expect if he has had a
distinct and altogether different origin? "The reply," says Professor
Huxley, "is not doubtful for a moment. Without question, the mode of
origin and the early stages of the development of man are identical with
those of the animals immediately below him in the scale." And again he
tells us: "It is very long before the body of the young human being can
be readily discriminated from that of the young puppy; but at a
tolerably early period the two become distinguishable by the different
forms of their adjuncts, the yelk-sac and the allantois;" and after
describing these differences he continues: "But exactly in those
respects in which the developing man differs from the dog, he resembles
the ape.... So that it is only quite in the latter stages of development
that the young human being presents marked differe