uccessive conditions of motion. B1 to B8, ditto of a Hermit-Crab
(_Chondracanthus cornutus_), in eight successive stages (after E.
von Beneden). C1 to C5, ditto of a Cat, in five successive stages
(after Pflueger). D, ditto of Trout; E, of a Hen; F, of Man. The
first series is taken from the _Encycl. Brit._; the second from
Haeckel's _Evolution of Man_.]

[Illustration: FIG. 32.--Human ovum, mature and greatly magnified.
(After Haeckel.)]

In thus saying that the ova of all animals are, so far as microscopes
can reveal, _substantially_ similar, I am of course speaking of the
egg-cell proper, and not of what is popularly known as the egg. The egg
of a bird, for example, is the egg-cell, _plus_ an enormous aggregation
of nutritive material, an egg-shell, and sundry other structures suited
to the subsequent development of the egg-cell when separated from the
parent's body. But all these accessories are, from our present point of
view, accidental or adventitious. What we have now to understand by the
ovum, the egg, or the egg-cell, is the microscopical germ which I have
just described. So far then as this germ is concerned, we find that all
multicellular organisms begin their existence in the same kind of
structure, and that this structure is anatomically indistinguishable
from that of the permanent form presented by the lowest, or unicellular
organisms. But although anatomically indistinguishable, physiologically
they present the sundry peculiarities already mentioned.

Now I have endeavoured to show that none of these peculiarities are such
as to exclude--or even so much as to invalidate--the supposition of
developmental continuity between the lowest egg-cells and the highest
protozoal cells. It remains to show in this place, and on the other
hand, that there is no breach of continuity between the lowest and the
highest egg-cells; but, on the contrary, that the remarkable uniformity
of the complex processes whereby their peculiar characters are exhibited
to the histologist, is such as of itself to sustain the doctrine of
continuity in a singularly forcible manner. On this account, therefore,
and also because the facts will again have to be considered in another
connexion when we come to deal with Weismann's theory of heredity, I
will here briefly describe the processes in question.

We have already seen that the young egg-cell multiplies itself by simple
binary division, after the manner of