development finds, not only that the chick commences its existence as
an egg, primarily identical, in all essential respects, with that of
the Dog, but that the yelk of this egg undergoes division--that the
primitive groove arises, and that the contiguous parts of the germ are
fashioned, by precisely similar methods, into a young chick, which, at
one stage of its existence, is so like the nascent Dog, that ordinary
inspection would hardly distinguish the two.

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 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 embryo 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.

Thus the study of development affords a clear test of closeness of
structural affinity, and one turns with impatience to inquire what