.--Ideal diagram, of primitive gill-or
aortic-arches. (After Rathke.) H, outline of heart. The arrows show
the course of the blood.]

[Illustration: FIG. 55.--The same, modified for a bird. (After Le
Conte.) The dark lines show the aortic arches which persist. A,
aorta; _p_, pulmonary arches; SC, S'C', sub-clavian; C, C',
carotids.]

[Illustration: FIG. 56.--The same, modified for a mammal. (After Le
Conte.)]

[Illustration: FIG. 57.--A series of embryos at three comparable and
progressive stages of development (marked I, II, III), representing
each of the classes of vertebrated animals below the Mammalia (After
Haeckel.)]

[Illustration: FIG. 58.--Another series of embryos, also at three
comparable and progressive stages of development (marked I, II,
III), representing four different divisions of the class Mammalia.
(After Haeckel.)]

If space permitted, it would be easy to present abundance of additional
evidence to the same effect from the development of the skeleton, the
skull, the brain, the sense-organs, and, in short, of every constituent
part of the vertebrate organization. Even without any anatomical
dissection, the similarity of all vertebrated embryos at comparable
stages of development admits of being strikingly shown, if we merely
place the embryos one beside the other. Here, for instance, are the
embryos of a fish, a salamander, a tortoise, a bird, and four different
mammals. In each case three comparable stages of development are
represented. Now, if we read the series horizontally, we can see that
there is very little difference between the eight animals at the
earliest of the three stages represented--all having fish-like tails,
gill-slits, and so on. In the next stage further differentiation has
taken place, but it will be observed that the limbs are still so
rudimentary that even in the case of Man they are considerably shorter
than the tail. But in the third stage the distinctive characters are
well marked.

* * * * *

So much then for an outline sketch of the main features in the embryonic
history of the Vertebrata. But it must be remembered that the science of
comparative embryology extends to each of the other three great branches
of the tree of life, where these take their origin, through the worms,
from the still lower, or gastraea, forms. And in each of these three
great branches--namely, th