THE PARTS OF BODIES 
7 
in the following respects: each one has a clear outer portion 
called the cell wall which incloses a mass of substance known 
as protoplasm (pro'td-plaz’m: Greek, protos, first; plasma, 
form). The protoplasm is made up of a substance called 
cytoplasm (si'to-plaz’m : Greek, kytos, hollow place ; plasma , 
form), in which is held a saclike 
body, the nucleus (nu'kle-us: 
Latin, nucleus, kernel, nut). The 
nucleus usually contains one or 
more separate bodies called 
nucleoli (nu kle'o-ll). A cell is 
therefore defined as a mass of 
protoplasm composed of cytoplasm 
and nucleus (Figure 4). 
2. Tissues. — The cells are of 
many shapes and sizes, and in 
the bodies of all but microscopic 
plants and animals they are united 
to help the plant or animal carry 
on its life processes. This union 
of cells to do a certain work is 
called a tissue, and the usual 
definition is: a tissue is a group 
of similar cells that do a similar 
work (Figure 5). 
3. Organs. — In all the higher 
animals the tissues are united into 
skin, arms, stomach, and so on, or 
in plants into leaf, branch, etc. Such structures are called 
organs ; an organ is defined as a group of tissues that do a 
given work in the animal or plant. 
4. The Organ System. — When different organs com¬ 
bine to carry on such a general life process as digestion, 
all the parts that assist in this process are described as an 
organ system, as the system of digestive organs. 
Identify the different parts 
from the description in the text. 
Compare the parts and shape 
of this cell with the cells shown 
in Figure 5. What conclusions 
can you draw ? 
