THE CELL 241 
situated and approximately spherical in form. The 
protoplasm is divided into an outer hyaline and an 
inner granular portion, the former being limited exter- 
nally by a very delicate membrane. The shape of the 
animal is irregular, and, moreover, undergoes gradual 
alteration owing to the characteristic amceboid move- 
ments. These consist in a slow protrusion and with- 
drawal of processes of the body, enabling the animal 
to change its position by a kind of flowing movement, 
and also to engulf its food, which consists of various 
Fic. 17.—AMCEBA. Fic. 18.—PLEUROCOCCUS, 
n, Nucleus; f, food particle. n, Nucleus; w, cell-wall ; 
chl, chloroplast. 
minute organic particles, by the simple process of 
flowing around it. 
In contrast with Ameba the unicellular plant Pleuro- 
coccus is motionless, and is surrounded by a firm wall 
of cellulose. In addition to a central nucleus, the 
plant contains, embedded in its peripheral protoplasm, 
several plastids which bear the chlorophyll concerned 
in the assimilation of carbon from the gases of the 
atmosphere. This chlorophyll lends a green colour to 
the whole contents of the cell, and in its natural habitat 
the plant is quite conspicuous. The green powdery 
substance often to be seen on the bark of trees, 
16 
