240 RECENT CYTOLOGY 
bodies or flastids. Certain plastids present in the 
majority of plants are of particular importance as con- 
taining the green substance chlorophyll, which plays 
an essential part in the fixation of carbon from the 
atmosphere. 
Amongst unicellular organisms—the creatures 
already mentioned as being made up of a single cell 
only—those which contain chlorophyll and are pro- 
vided with a firm cell wall, built up of a material 
known as cellulose, are usually regarded as simple 
plants; whilst those in which chlorophyll and a cell 
wall are absent are looked upon as simple types of 
animals. Similarly slight differences distinguish the 
cells which build up the fabric of the higher plants 
from those of which the bodies of the more complicated 
animals are composed, so that in almost all essential 
points an account of the behaviour of the cells of the 
members of one kingdom will apply equally well to 
those of the other. After a few further preliminary 
remarks we shall, therefore, for the sake of simplicity, 
speak of a generalized type of cell, the behaviour of 
which, except in points of detail, will resemble that of 
the actual cells of plants or animals indifferently. But 
in order to convey a more definite idea of an uni- 
cellular animal to those who are unfamiliar with the 
rudiments of Elementary Biology, we may refer briefly 
to the well-known form Ameba, which will serve as 
an excellent type of an animal consisting of a single 
free-living cell. 
This little creature consists of a mass of protoplasm 
enclosing a nucleus which is more or less centrally 
