192 THEORY OF THE CELLS. 
tially similar laws; and, therefore, that these processes must, 
in every instance, be produced by the same powers. Now, if we 
find that some of these elementary parts, not differing from 
the others, are capable of separating themselves from the 
organism, and pursuing an independent growth, we may thence 
conclude that each of the other elementary parts, each cell, 
is already possessed of power to take up fresh molecules and 
grow; and that, therefore, every elementary part possesses a 
power of its own, an independent life, by means of which it 
would be enabled to develop itself independently, if the relations 
which it bore to external parts were but similar to those in 
which it stands in the organism. The ova of animals afford 
us examples of such independent cells, growing apart from the 
organism. It may, indeed, be said of the ova of higher animals, 
that after impregnation the ovum is essentially different from 
the other cells of the organism; that by impregnation there 
is a something conveyed to the ovum, which is more to it than 
an external condition for vitality, more than nutrient matter ; 
and that it might thereby have first received its peculiar 
vitality, and therefore that nothing can be inferred from it with 
respect to the other cells. But this fails in application to those 
classes which consist only of female individuals, as well as with 
the spores of the lower plants ; and, besides, in the inferior 
plants any given cell may be separated from the plant, and 
then grow alone. So that here are whole plants consisting 
of cells, which can be positively proved to have independent 
vitality. Now, as all cells grow according to the same laws, 
and consequently the cause of growth cannot in one case lie 
in the cell, and in another in the whole organism ; and since 
it may be further proved that some cells, which do not differ 
from the rest in their mode of growth, are developed indepen- 
dently, we must ascribe to all cells an independent vitality, that 
is, such combinations of molecules as occur in any single cell, 
are capable of setting free the power by which it is enabled 
to take up fresh molecules. The cause of nutrition and 
growth resides not in the organism as a whole, but in the 
separate elementary parts—the cells. The failure of growth 
in the case of any particular cell, when separated from an 
organized body, is as slight an objection to this theory, as it 
is an objection against the independent vitality of a bee, that 
