membrane; but the white,
the membrane, and the shell have all the same quality, except that the
proportion of lime is more or less in the different layers.

But, as I have said, the various envelopes of eggs, the presence or
absence of a shell, and the absolute size of the egg, are accessory
features, belonging not to the egg as egg, but to the special kind of
being from which the egg has arisen and into which it is to develop.
What is common to all eggs and essential to them all is that which
corresponds to the yolk in the bird's egg. But their later mode of
development, the degree of perfection acquired by the egg and germ
before being laid, the term required for the germ to come to maturity,
as well as the frequency and regularity of the broods, are all features
varying with the different kinds of animals. There are those that lay
eggs once a year at a particular season and then die; so that their
existence may be compared to that of annual plants, undergoing their
natural growth in a season, to exist during the remainder of the year
only in the form of an egg or seed. The majority of Insects belong to
this category, as do also our large Jelly-Fishes; many others have a
slow growth, extending over several years, during which they reach their
maturity, and for a longer or shorter time produce broods at fixed
intervals; while others, again, reach their mature state very rapidly,
and produce a number of successive generations in a comparatively short
time, it may be in a single season.

I do not intend to enter upon the chapter of special differences of
development among animals, for in this article I have aimed only to show
that the egg lives, that it is itself the young animal, and that
the vital principle is active in it from the earliest period of its
existence. But I would say to all young students of Embryology that
their next aim should be to study those intermediate phases in the life
of a young animal, when, having already acquired independent existence,
it has not yet reached the condition of the adult. Here lies an
inexhaustible mine of valuable information unappropriated, from which,
as my limited experience has already taught me, may be gathered the
evidence for the solution of the most perplexing problems of our
science. Here we shall find the true tests by which to determine the
various kinds and different degrees of affinity which animals now living
bear not only to one another, but also to those that h