ter in the case of all Protozoa, is
sufficient to show that the distinction now before us--like the one last
dealt with--is by no means absolute. As in the case of sexual
propagation, so in that of karyokinesis, processes which are common to
all the Metazoa are not wholly without their foreshadowings in the
Protozoa. And seeing how greatly exalted is the office of egg-cells--and
even of tissue-cells--as compared with that of their supposed ancestry
in protozoal cells, it seems to me scarcely to be wondered at if their
specializations of function should be associated with corresponding
peculiarities of structure--a general fact which would in no way
militate against the doctrine of evolution. Could we know the whole
truth, we should probably find that in order to endow the most primitive
of egg-cells with its powers of marshalling its products into a living
army of cell-battalions, such an egg-cell must have been passed through
a course of developmental specialization of so elaborate a kind, that
even the complex processes of karyokinesis are but a very inadequate
expression thereof.

Probably I have now said enough to show that, remarkable and altogether
exceptional as the properties of germ-cells of the multicellular
organisms unquestionably show themselves to be, yet when these
properties are traced back to their simplest beginnings in the
unicellular organisms, they may fairly be regarded as fundamentally
identical with the properties of living cells in general. Thus viewed,
no line of real demarcation can be drawn between growth and
reproduction, even of the sexual kind. The one process is, so to speak,
physiologically continuous with the other; and hence, so far as the
pre-embryonic stage of life-history is concerned, the facts cannot
fairly be regarded as out of keeping with the theory of evolution.

I will now pass on to consider the embryogeny of the Metazoa, beginning
at its earliest stage in the fertilization of the ovum. And here it is
that the constructive argument in favour of evolution which is derived
from embryology may be said properly to commence. For it is surely in
itself a most suggestive fact that all the Metazoa begin their life in
the same way, or under the same form and conditions. _Omne vivum ex
ovo._ This is a formula which has now been found to apply throughout the
whole range of the multicellular organisms. And seeing, as we have just
seen, that the ovum is everywhere a single cell, the for