Omne vivum ex ovo,"--yet he was not himself aware of the
significance of his own statement, for the existence of the Mammalian
egg was not then dreamed of. Since then the discoveries of von Baer and
others have shown not only that the egg is common to all living beings
without exception, from the lowest Radiate to the highest Vertebrate,
but that its structure is at first identical in all, composed of the
same primitive elements and undergoing exactly the same process of
growth up to the time when it assumes the special character peculiar
to its kind. This is unquestionably one of the most comprehensive
generalizations of modern times.

In common parlance, we understand by an egg something of the nature of a
hen's egg, a mass of yolk surrounded with white and inclosed in a shell.
But to the naturalist, the envelopes of the egg, which vary greatly in
different animals, are mere accessories, while the true egg, or, as it
is called, the ovarian egg, with which the life of every living being
begins, is a minute sphere, uniform in appearance throughout the Animal
Kingdom, though its intimate structure is hardly to be reached even with
the highest powers of the microscope. Some account of the earlier stages
of growth in the egg may not be uninteresting to my readers. I will
take the egg of the Turtle as an illustration, since that has been the
subject of my own especial study; but, as I do not intend to carry my
remarks beyond the period during which the history of all vertebrate
eggs is the same, they may be considered of more general application.

It is well known that all organic structures, whether animal or
vegetable, are composed of cells. These cells consist of an outside bag
inclosing an inner sac, and within that sac there is a dot. The outer
bag is filled with semi-transparent fluid, the inner one with a
perfectly transparent fluid, while the dot is dark and distinct. In the
language of our science, the outer envelope is called the Ectoblast, the
inner sac the Mesoblast, and the dot the Entoblast. Although they are
peculiarly modified to suit the different organs, these cells never lose
this peculiar structure; it may be traced even in the long drawn-out
cells of the flesh, which are like mere threads, but yet have their
outer and inner sac and their dot,--at least while forming.

In the Turtle the ovary is made up of such cells, spherical at first,
but becoming hexagonal under pressure, when they are more closely