4 ZOOLOGICAL EXERCISES. 
a cell. In vegetables, the corpuscles generally surround 
themselves with a cell wall of cellulose; in animals, the 
corpuscles usually remain naked, the external secretion, if 
any, being dissolved; nevertheless, for the sake of conve- 
nience, animal corpuscles of protoplasm are also called cells. 
As the cell increases in size by the introduction of new 
matter, which in the fluid state can permeate the cell wall, 
this structure is stretched, and would rupture if 1t were not 
that material of the same kind is constantly being added to 
it by intus-susception, that is, by the interposition of new 
molecules between the old ones, so that the cell wall grows 
too, and gets thicker. This mode of growth of organic 
bodies by intus-susception is altogether different from the 
growth of inorganic bodies, such as crystals, which grow 
by successively deposited external layers, and can only 
grow when material of the same substance as themselves is 
presented to them. 
Structure of Organic Substances.—Although we know 
what elements are present in living matter, and approxi- 
mately in what proportions they are present, we do not 
know whether they form a simple or complex combination. 
We cannot analyse living matter, as the first step in the 
process must always kill it; but in non-living, or secreted 
matter, a combination of the elements certainly takes place. 
Nageli has proposed the hypothesis that in organised bodies 
each molecule is surrounded by a layer of water. All these 
bodies contract by drying, and swell by imbibing moisture ; 
but the difficulty of receiving the hypothesis as true lies 1n 
the fact that organic substances may be exposed to a tempe- 
rature below freezing without becoming solid. As cell walls 
and starch swell or shrink most in certain directions, it 
would seem that the molecules composing them are not 
spherical, and as these substances depolarise light, 1t would 
