124 PHOTO-MICROGRAPHS. 
justifies, the enlargement of the work, it may 
eventually be given the title indicated. 
The general truth which these photo-micro- 
graphs are intended to illustrate is that the low- 
est living beings are UNICELLULAR ORGANISMS, and 
that the tissues of higher plants and animals are made 
up of CELLS. 
The essential part of a diving cell is a little mass 
of transparent jelly-like material called protoplasm. 
This is a complex nitrogenous substance, which 
varies within certain limits as to its consistence, 
the presence or absence of granules, nucle?, nucle- 
oli, etc.; but it is essentially the same both in 
animal and vegetable cells, so far as its chem- 
ical composition and reactions are concerned. Its 
vital potentiality, however, varies within wide 
limits. This is illustrated by the fact that one 
little mass of protoplasm enclosed in an envelope 
of cellulose may be restricted as to its power 
of development to the simple vé/e of producing 
other cells like the first, unicellular organisms; 
while another similar mass, enclosed in a mem- 
brane of different composition, — as, for example, 
the fertilized ovum of a mammal, — may develop 
into a vertebrate animal. 
Vegetable cells and most animal cells have a 
cell-wall, or limiting membrane. In vegetable cells, 
this is composed of cellulose, and is well defined. 
In animal cells, it consists of nitrogenous material, 
and is often extremely thin, and difficult to dem- 
onstrate. 
