* * *

Embryology as a distinct division of zooelogy has grown out of studies of
classification and comparative anatomy. Its beginnings may be found in
medieval natural history, for as far back as 1651 Harvey had pointed out
that all living things originate from somewhat similar germs, the terse
dictum being "Ex ovo omnia." By the end of the eighteenth century many had
turned to the study of developing organisms, though their views by no
means agreed as to the way an adult was related to the egg. Some, like
Bonnet, held that the germ was a minute and complete replica of its
parent, which simply unfolded and enlarged like a bud to produce a similar
organism. Even if this were true, little would be gained, for it would
still remain unknown how the germinal miniature originated to be just what
it was conceived and assumed to be. Wolff was the originator of the view
that is now practically universal among naturalists, namely, that
development is a real process of transformation from simpler to more
complex conditions.

The subject of comparative embryology grew rapidly during the nineteenth
century as the field of comparative anatomy became better known, and when
naturalists became interested in animals, not only as specific types, but
also as the finished products of an intricate series of transformations.
When life-histories were more closely compared, the meaning of the
resemblances between early stages of diverse adult organisms was read by
the same method which in comparative anatomy finds that consanguinity is
expressed by resemblance. The great law of recapitulation, stated in one
form by Von Baer and more definitely by Haeckel in the terms employed in
the foregoing sections, was for a time too freely used and too rigidly
applied by naturalists whose enthusiasm clouded their judgment. A strong
reaction set in during the latter part of the nineteenth century, when
attention was directed to the anachronisms of the embryonic record and to
the alterations that are the results of larval or embryonic adaptation as
short cuts in development. Nevertheless, it is not seriously questioned, I
believe, that the main facts of a single life-history owe their nature to
the past evolution of the species to which a given animal belongs.

Nowadays the problems in this well-organized department are concerned not
only with more accurate accounts of the development of animals, but also
with the mechanics of development, w