56 ANIMAL LIFE 
colonial Protozoa, or between members of different colonies 
of the same species. These conjugating individuals in the 
simpler kinds of colonies, like Goniwm, are similar; in 
Pandorina they appear to be slightly different, and in Hudo- 
rina and Volvox the conjugating cells are very different from 
each other (Figs. 15 and 16). One kind of cell, which is 
called the egg cell, is large, spherical, and inactive, while 
the other kind, the sperm cell, is small, with ovoid head 
and tapering tail, and free-swimming. In the simpler colo- 
nial Protozoa all the cells of the body take part in repro- 
duction, but in Volvox only certain cells perform this func- 
tion, and the other cells of the body die. Or we may say 
that the body of Volvox dies after it has produced special 
reproductive cells which shall fulfill the function of multi- 
plication. 
Beginning with the more complex Volvocinez, which we 
may call either the most complex of the one-celled animals 
or the simplest of the many-celled animals, all the complex 
animals show this distinct differentiation between the re- 
productive cells and the cells of the rest of the body. Of 
course, we find, as soon as we go up at all far in the scale of 
the animal world, that there is a great deal of differentia- 
tion among the cells of the body: the cells which have to 
do with the assimilation of food are of one kind; those on 
which depend the motions of the body are of another kind; 
those which take oxygen and those which excrete waste 
matter are of other kinds. But the first of this cell differ- 
entiation, as we have already often repeated, is that shown 
by the reproductive cells; and with the very first of this 
differentiation between reproductive cells and the other 
body cells appears a differentiation of the reproductive 
cells into two kinds. These two kinds, among all animals, 
are always essentially similar to the two kinds shown by 
Volvoz and the simplest of the many-celled animals—namely, 
large, inactive, spherical egg cells, and small, active, elon- 
gate or “ tailed ” sperm cells. 
