while there are found in the lowest realm of animal life, organisms like
the amoeba of stagnant pools, consisting of nothing more than minute
masses of protoplasm, there are others like them which possess a small
central body called a nucleus. This is known as nucleated protoplasm.

[Illustration: Fig. 1.--Diagram of a Cell.

A, nucleus;
B, nucleolus;
C, protoplasm. (Highly magnified)
]

11. Cells. When we carry back the analysis of an organized body as
far as we can, we find every part of it made up of masses of nucleated
protoplasm of various sizes and shapes. In all essential features these
masses conform to the type of protoplasmic matter just described. Such
bodies are called cells. In many cells the nucleus is finely granular or
reticulated in appearance, and on the threads of the meshwork may be one
or more enlargements, called nucleoli. In some cases the protoplasm at the
circumference is so modified as to give the appearance of a limiting
membrane called the cell wall. In brief, then, a cell is a mass of
nucleated protoplasm; the nucleus may have a nucleolus, and the cell
may be limited by a cell wall. Every tissue of the human body is formed
through the agency of protoplasmic cells, although in most cases the
changes they undergo are so great that little evidence remains of their
existence.

There are some organisms lower down in the scale, whose whole activity is
confined within the narrow limits of a single cell. Thus, the amoeba
begins its life as a cell split off from its parent. This divides in its