erical wall of the ovum where before there had only been
one. Moreover, we have also seen that a precisely similar series of
events repeat themselves in each of these two cells, thus giving rise to
four cells (see Fig. 29). It must now be added that such duplication is
continued time after time, as shown in the accompanying illustrations
(Figs. 39, 40).

[Illustration: FIG. 39.--Segmentation of ovum. (After Haeckel.)
Successive stages are marked by the letters A, B, C. D represents
several stages in advance of C.]

[Illustration: FIG. 40.--The contents of an ovum in an advanced
stage of segmentation, drawn in perspective. (After Haeckel.)]

All this, it will be noticed, is a case of cell-multiplication, which
differs from that which takes place in the unicellular organisms only in
its being _invariably_ preceded (as far as we know) by karyokinesis, and
in the resulting cells being all confined within a common envelope, and
so in not being free to separate. Nevertheless, from what has already
been said, it will also be noticed that this feature makes all the
difference between a Metazooen and a Protozooen; so that already the ovum
presents the distinguishing character of a Metazooen.

I have dealt thus at considerable length upon the processes whereby the
originally unicellular ovum and spermatozooen become converted into the
multicellular germ, because I do not know of any other exposition of the
argument from Embryology where this, the first stage of the argument,
has been adequately treated. Yet it is evident that the fact of all the
processes above described being so similar in the case of sexual (or
metazoal) reproduction among the innumerable organisms where it occurs,
constitutes in itself a strong argument in favour of evolution. For the
mechanism of fertilization, and all the processes which even thus far we
have seen to follow therefrom, are hereby shown to be not only highly
complex, but likewise highly specialized. Therefore, the remarkable
similarity which they present throughout the whole animal kingdom--not
to speak of the vegetable--is expressive of organic continuity, rather
than of absolute discontinuity in every case, as the theory of special
creation must necessarily suppose. And it is evident that this argument
is strong in proportion to the uniformity, the specialization, and the
complexity of the processes in question.

Having occupied so much space with supplying what appea