44 THE ELEMENTS OF STRUCTURE. 
IV. Cells.—In discussing tissues, it was necessary to 
refer to the component cells. Let us now consider the 
chief characteristics of these elements. 
A cell is a unit mass or area of living matter usually with 
a nucleus. Most of the simplest animals and plants 
(Protozoa and Protophyta) are single cells; eggs and male 
elements are single cells; in multicellular organisms the 
cells are combined into tissues and organs. 
Most cells are too 
small to be distinguished 
except through lenses; 
many Protozoa, ¢.g. large 
Ameebee, are just visible 
to our unaided eyes; the 
chalk -forming Foramin- 
ifera are single cells, whose 
shells are often as ‘large 
as pin-heads, and some of 
the extinct kinds were as 
big as half-crowns (see 
Fig. 17); the bast cells 
of plants may extend for 
i several inches; the largest 
Fic. 21.—Diagram of cell structure. animal cells are eggs dis- 
atte WAG Rs tended with yolk. 
Pi. Plastids in cytoplasm. The typical and primi- 
cc. Centrosome. 
ate Beclealis tive form of cell is a 
‘W, Nucleus sphere—a shape naturally 
ct. General cytoplasm. assumed by a complex 
V. Vacuole. f 
Gr. Granules. coherent substance situ- 
ated in a medium different 
from itself. Most egg-cells and many Protozoa retain this 
primitive form, but the internal and external conditions of 
life (such as nutrition and pressure) often evolve other 
shapes,—oval, rectangular, flattened, thread-like, stellate, 
and so on. 
As to the structure of a cell, we may distinguish (see 
Fig. 21)— 
(a) The general cell substance or cytoplasm, which con- 
sists partly of genuinely living stuff or protoplasm, and 
partly of complex materials not really living (metaplasm) ; 
