FIG. 12.--A. Egg of the Dog, with the vitelline membrane burst, so as
to give exit to the yolk, the germinal vesicle (a), and its included
spot (b). B. C. D. E F. Successive changes of the yolk indicated in
the text. After Bischoff.

The oak is a more complex thing than the little rudimentary plant
contained in the acorn; the caterpillar is more complex than the egg;
the butterfly than the caterpillar; and each of these beings, in
passing from its rudimentary to its perfect condition, runs through a
series of changes, the sum of which is called its Development. In the
higher animals these changes are extremely complicated; but, within the
last half century, the labours of such men as Von Baer, Rathke,
Reichert, Bischof, and Remak, have almost completely unravelled them, so
that the successive stages of development which are exhibited by a Dog,
for example, are now as well known to the embryologist as are the steps
of the metamorphosis of the silkworm moth to the school-boy. It will
be useful to consider with attention the nature and the order of the
stages of canine development, as an example of the process in the
higher animals generally.

The Dog, like all animals, save the very lowest (and further inquiries
may not improbably remove the apparent exception), commences its
existence as an egg: as a body which is, in every sense, as much an egg
as that of a hen, but is devoid of that accumulation of nutritive
matter which confers upon the bird's egg its exceptional size and
domestic utility; and wants the shell, which would not only be useless
to an animal incubated within the body of its parent, but would cut it
off from access to the source of that nutriment which the young
creature requires, but which the minute egg of the mammal does not
contain within itself.