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.
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 silk-worm moth to the schoolboy. 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.