means of successive
cell-divisions, which, beginning in the fertilized ovum, eventually
build up all the tissues and organs of the body.

[Illustration: FIG. 28.--_Hydra viridis_, partly in section. M,
mouth; O, ovary, or bud containing female reproductive cells; T,
testis, or bud containing male reproductive cells. In addition to
these buds containing germinal elements alone, there is another
which illustrates the process of "gemmation"--i. e. the direct
out-growth of a fully formed offspring.]

This process clearly indicates very high specialization on the part of
germ-cells. For we see by it that although these cells when young
resemble all other cells in being capable of self-multiplication by
binary division (thus reproducing cells exactly like themselves), when
older they lose this power; but, at the same time, they acquire an
entirely new and very remarkable power of giving rise to a vast
succession of many different kinds of cells, all of which are mutually
correlated as to their several functions, so as to constitute a
hierarchy of cells--or, to speak literally, a multicellular
_co-organization_. Here it is that we touch the really important
distinction between the Protozoa and the Metazoa; for although I have
said that some of the higher Protozoa foreshadow this state of matters
in forming cell-colonies, it must now be noted that the cells composing
such colonies are all of the same kind; and, therefore, that the
principle of producing different kinds of cells which, by mutual
co-adaptation of functions, shall be capable of constructing a
multicellular Metazooen,--this great principle of _co-organization_ is
but dimly nascent in the cell-colonies of Protozoa. And its marvellous
development in the Metazoa appears ultimately to depend upon the highly
specialized character of germ-cells. Even in cases where multicellular
organisms are capable of reproducing their kind without the need of any
preceding process of fertilization (parthenogenesis), and even in the
still more numerous cases where complete organisms are budded forth from
any part of their parent organism (gemmation, Fig. 28), there is now
very good reason to conclude that these powers of a-sexual reproduction
on the part of multicellular organisms are all ultimately due to the
specialized character of their germ-cells. For in all these cases the
tissues of the parent, from which the budding takes place, were
ultimately derive