162 GENERAL RETROSPECT.
figure, especially in animals ; in plants they are, for the most
part or exclusively, cells. This variety in the elementary
parts seemed to hold some relation to their more diversified
physiological function in animals, so that it might be established
as a principle, that every diversity in the physiological signification of an organ requires a difference in its elementary particles ; and, on the contrary, the similarity of two elementary particles seemed to justify the conclusion that they were
physiologically similar. It was natural that among the very
different forms presented by the elementary particles, there
should be some more or less alike, and that they might be
divided, according to their similarity of figure, into fibres, which
compose the great mass of the bodies of animals, into cells, tubes, globules, &c. The division was, of course, only one of
natural history, not expressive of any physiological idea, and
just as a primitive muscular fibre, for example, might seem to
differ from one of areolar tissue, or all fibres from cells, so would
there be in like manner a difference, however gradually
marked between the different kinds of cells. It seemed as if
the organism arranged the molecules in the definite forms
exhibited by its different elementary particles, in the way required by its physiological function. It might be expected that there would be a definite mode of development
for each separate kind of elementary structure, and that it
would be similar in those structures which were physiologically identical, and such a mode of development was, indeed, already more or less perfectly known with regard to
muscular fibres, blood-corpuscles, the ovum (see the Supplement), and epithelium-cells. The only process common to
all of them, however, seemed to be the expansion of their
elementary particles after they had once assumed their proper
form. The manner in which their different elementary particles were first formed appeared to vary very much. In
muscular fibres they were globules, which were placed together
im rows, and coalesced to form a fibre, whose growth proceeded
in the direction of its length. In the blood-corpuscles it was
a globule, around which a vesicle was formed, and continued
to grow; in the case of the ovum, it was a globule, around
which a vesicle was developed and continued to grow, and
around his again a second vesicle was formed.
