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 signi- 
fication of an organ requires a difference in its elementary 
particles ; and, on the contrary, the similarity of two elemen- 
tary 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, &e. 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 ex- 
pected 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 physiologi- 
cally identical, and such a mode of development was, in- 
deed, already more or less perfectly known with regard to 
muscular fibres, blood-corpuscles, the ovum (see the Supple- 
ment), 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 par- 
ticles 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. 
