CHAPTER IX 
RECENT CYTOLOGY 
Every living creature may be regarded as being built 
up of a number of structural units which are known 
as cells. In the case of some of the simplest animals 
and plants, indeed, the whole body of the organism is 
composed of a single cell—a small mass of living proto- 
plasm, containing, as a rule, only one nucleus. But 
in all the higher animals and plants the adult body is 
made up of a great number of such cells living in 
intimate association with one another. 
The living material of which the cell is composed is 
known as protoplasm. Protoplasm is a highly com- 
plicated and unstaple combination of substances, 
amongst the constituents of which the chemical 
elements, carbon, oxygen, hydrogen, nitrogen, and 
sulphur, play the chief parts. Its consistency is slimy 
and semifluid. 
Concerning the nucleus—the most essential and 
characteristic of cell organs—more will have to be said 
later on. Other important organs of cells are a wall 
or membrane which externally surrounds them, one 
or more vacuoles or cavities containing a watery fluid, 
or sometimes a gas, and a certain number of more solid 
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