reptiles and birds are distinctly different from
mammals, the dog and the man are almost identical.

The gill-arches of the fish exist in man, in dogs, in fowls, in
reptiles, and in other vertebrate animals during the first stages of
their development. Man also possesses, in his first stages, a real tail,
as well as his nearest kindred--the tailless apes (orang-outang,
chimpanzee, gorilla), and vertebrate animals in general. The tail, as
has been stated, man still retains, though hidden as a rudiment.

[Illustration: FIG. I.--Human Embryo.--_Ecker._]

[Illustration: FIG. II.--Embryo of Dog.--_Bischoff._]

[Illustration: FIG. III.--Dog Embryo.--_Huxley._]

[Illustration: FIGS. IV, V, and VI.--Embryo of Rabbit in three stages of
development.--_Haeckel._]

[Illustration: FIGS. VII, VIII, and IX.--Embryo of Man in three stages
of development.--_Haeckel._ _v_, fore brain; _z_, twix brain; _m_,
middle brain; _h_, hind brain; _n_, after brain; _r_, spinal marrow;
_e_, nose; _a_, eye; _o_, ear; _k_, gillarches; _g_, heart; _w_,
vertebral column; _f_, fore limbs; _b_, hind limbs; _s_, tail.]

"Man presents in his earliest stages of embryonic growth, a skeleton of
cartilage, like that of the lamprey; also, five origins of the aorta and
five slits on the neck, like the _lamprey_ and the _shark_. Later, he
has but four aortic origins, and a heart now divided into two chambers,
like _bony fishes_; the optic lobes of his brain also having a very
fish-like predominance in size. Three chambers of the heart and three
aortic origins follow, presenting a condition permanent in the
_batrachia_; then two origins with enlarged hemispheres of the brain, as
in _reptiles_. Four heart chambers and one aortic root on each side,
with slight development of the cerebellum, agree with the characters of
the _crocodiles_, and immediately present the special mammalian
conditions, single aortic root, and the full development of the
cerebellum. Later comes that of the cerebrum, also in its higher
mammalian or human traits." At no time in the development of the egg,
save at the start, do the embryos of the various vertebra assume the
_exact_ or _entire_ characteristics of one another, but they assimilate
so closely that it requires the eye of the expert to distinguish them;
and, as has already been stated, the more closely an animal resembles
another, the longer and the more intimately do their embryos resemble
one another; so that, for example