THE CELL. 5 
THE CELL.* 
All living things, great and small, are composed of cells. 
Animals may be divided into those consisting of a single cell 
(Protozoa), and those made up of a multitude of cells (Metazoa) ; 
but in every case the animal begins as a single cell or ovum 
from which all the other cells, however different finally from 
one another either in form or function, are derived by processes 
of growth and division; and, as will be seen later, the whole 
organism is at one period made up of cells practically alike in 
structure and behavior. The history of each individual animal 
or plant is the resultant of the conjoint histories of each of its 
cells, as that of a nation is, when complete, the story of the 
total outcome of the lives of the individuals composing it. 
It becomes, therefore, highly important that a clear notion 
of the characters of the cell be obtained at the outset; and 
this chapter will be devoted to presenting a general account of 
the cell. 
The cell, whether animal or vegetable, in its most complete 
form consists of a mass of viscid, semifluid, transparent sub- 
stance (protoplasm), a cell wall, and a more or less circular body 
(nucleus) situated generally centrally within; in which, again, 
is found a similar structure (nucleolus). 
This description applies to both the vegetable and the ani- 
mal cell; but the student will find that the greater proportion 
of animal cells have no cell wall, and that very few vegetable 
cells are without it. But there is this great difference between 
the animal and vegetable cell: the former never has a cellulose 
wall, while the latter rarely lacks such a covering. In every 
case the cell wall, whether in animal or vegetable cells, is of 
greater consistence than the rest of the cell. This is especially 
true of the vegetable cell. 
It is doubtful whether there are any cells without a nucleus, 
while not a few, especially when young and most active, pos- 
sess several. The circular form may be regarded as the typical 
form of both cells and nuclei, and their infinite variety in size 
and form may be considered as in great part the result of the 
action of mechanical forces, such as mutual pressure; this is,. 
of course, more especially true of shape. Reduced to its great- 
est simplicity, then, the cell may be simply a mass of protoplasm 
with a nucleus. 
* The illustrations of the sections following will enable the student to form a 
generalized mental picture of the cell in all its parts. 
