CELL THEORIES. 117 
The history of conceptions regarding cells is 
in its general outlines exceedingly instructive. 
The notion of a cell was first derived from vege- 
table tissues with their easily exhibited cellulose 
cell-walls. The vesicular form thus caught the eye 
from the first. Then, in many instances in animal 
tissues also a real or apparent vesicular structure 
was easily observed ; as in adipose, epithelial, and 
nerve-corpuscles. The contents in vesicular struc- 
tures were seen to be various, but the frequent 
existence of a nucleus of firmer consistence im- 
bedded in them could be demonstrated, and within 
this were often seen one or more nucleoli of some 
sort or other. Thus it naturally happened that 
the cell-wall was considered a characteristic struc- 
ture, and was supposed to be functionally import- 
ant; and next to it, the nucleus was regarded 
as the seat of vital properties, because it was 
seen to divide preparatory to the multiplication 
of cells, and to be distinct in young cells, however 
it might dwindle out of sight in the old. The 
circumstance, manifest from the outset, that cell 
contents were various, taken in conjunction with 
their being the part which least caught the eye, 
led to their vital importance being long overlooked, 
But as improved microscopic methods came into 
use, including reagents, such as carmine, which 
