correspondences disclosed by the earliest stages of development. But in
spite of their complexity, all the changes of "growing up" are explained
and understood by the simple formula that the mode of individual
development owes its nature primarily to the hereditary influence of
earlier ancestors back to the original animals which were protozoa.

* * * * *

Embryology as a distinct division of zoölogy 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