hether cell-division is fundamentally identical in unicellular
and in multicellular organisms.

[9] I say "probably," because analogy points in this direction. As a
matter of fact, in many cases of tissue-formation karyokinesis has
not hitherto been detected. But even if in such cases it does not
occur--i. e. if failure to detect its occurrence be not due merely
to still remaining imperfections of our histological methods,--the
large number of cases in which it has been seen to occur in the
formation of sundry tissues are of themselves sufficient to indicate
some important difference between cells derived from ova (metazoal),
and cells which have not been so derived (protozoal). Which is the
point now under discussion.

[Illustration: FIG. 29.--Successive stages in the division of the
ovum, or egg-cell, of a worm. (After Strasburger.) _a_ to _d_ show
the changes taking place in the nucleus and surrounding
cell-contents, which result in the first segmentation of the ovum at
_e_; _f_ and _g_ show a repetition of these changes in each of the
two resulting cells, leading to the second segmentation stage at
_h_.]

Lastly, the only other distinction of a physiologically significant kind
between a single cell when it occurs as a Protozooen and when it does so
as the unfertilized ovum of a Metazooen is, that in the latter case the
nucleus discharges from its own substance two minute protoplasmic masses
("polar bodies"), which are then eliminated from the cell altogether.
This process, which will be more fully described later on, appears to be
of invariable occurrence in the case of all egg-cells, while nothing
resembling it has ever been observed in any of the Protozoa.

We must now consider these several points of difference _seriatim_.

First, with regard to sexual propagation, we have already seen that this
is by no means the only method of propagation among the multicellular
organisms; and it now remains to add that, on the other hand, there is,
to say the least, a suggestive foreshadowing of sexual propagation among
the unicellular organisms. For although simple binary fission is here
the more usual mode of multiplication, very frequently two (rarely three
or more) Protozoa of the same species come together, fuse into a single
mass, and thus become very literally "one flesh." This process of
"conjugation" is usually (though by no means invariably) foll