have been made known by the
study of Development.

It is a truth of very wide, if not of universal, application, that every
living creature commences its existence under a form different from, and
simpler than, that which it eventually attains.

(FIGURE 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,
Bischoff, 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.

The Dog's egg is, in fact, a little spheroidal bag (Figure 12), formed
of a delicate transparent membrane called the 'vitelline membrane', and
about 1/130 to 1/120th of an inch in diameter. It contains a mass of
viscid nutritive matter--the 'yelk'--within which is inclosed a second
much more delicate spheroidal bag, called the 'germinal vesicle' (a). In
this, lastly, lies a more solid rounded body, termed t