Here I ought to say a word about the important part played by the
chick in the growth of our science. The development of the chick, like
that of the young of all other birds, agrees in all its main features
with that of the other chief vertebrates, and even of man. The three
highest classes of vertebrates--mammals, birds, and reptiles (lizards,
serpents, tortoises, etc.)--have from the beginning of their embryonic
development so striking a resemblance in all the chief points of
structure, and especially in their first forms, that for a long time
it is impossible to distinguish between them. We have known now for
some time that we need only examine the embryo of a bird, which is the
easiest to get at, in order to learn the typical mode of development
of a mammal (and therefore of man). As soon as scientists began to
study the human embryo, or the mammal-embryo generally, in its earlier
stages about the middle and end of the seventeenth century, this
important fact was very quickly discovered. It is both theoretically
and practically of great value. As regards the THEORY of evolution, we
can draw the most weighty inferences from this similarity between the
embryos of widely different classes of animals. But for the practical
purposes of embryological research the discovery is invaluable,
because we can fill up the gaps in our imperfect knowledge of the
embryology of the mammals from the more thoroughly studied embryology
of the bird. Hens' eggs are easily to be had in any quantity, and the
development of the chick may be followed step by step in artificial
incubation. The development of the mammal is much more difficult to
follow, because here the embryo is not detached and enclosed in a
large egg, but the tiny ovum remains in the womb until the growth is
completed. Hence, it is very difficult to keep up sustained
observation of the various stages in any great extent, quite apart
from such extrinsic considerations as the cost, the technical