6 ZOOLOGY. 
well as the entire Ameba or monad, is complex. It consists 
of carbon, hydrogen, oxygen, nitrogen, and sulphur, combined 
in nearly the same proportions. The protoplasm of different 
cells exerts widely different forces and capabilities. An egg- 
cell becomes a man, whose brain-cells are the medium of the 
intellectual power which enables him to write the history of 
his own species, and to be the historian of the forms of life 
which stand below him. The cell is the morphological 
unit of the organic world. With cells the biologist can 
in the imagination reconstruct the vegetable and animal 
worlds. 
The primitive form of a cell, when without a nucleus or 
nucleolus, is called a cytode ; genuine cells have a nucleus, 
the latter containing a nucleolus. Animals composed of but a 
single cell, such as the Ameba or an Infusorian, are said to be 
unicallular. Cells grow by absorbing cell-food—i.e., by the 
assimilation of matter from without, and this matter may be 
in masses of considerable size when seen under the microscope. 
Cells multiply by self-division. The egg-cell undergoes 
division of the yolk into two, four, 
eight, and afterward many cells; the 
cells thus formed become arranged into 
the inner the endoderm. A third germ- 
layer arises between them, called the 
Fig. 8.—Germ of Sagitta, mesoderm or middle germ-layer. From 
ester totum these germ-layers, or cell-layers, the 
cleated celle. tissues of the body are formed, such as 
muscle, bone, nerve, and glandular tissue. These tissues 
form organs, hence animals (as well as plants) are called or- 
ganisms, because they have certain parts formed of a partic- 
ular kind of tissue set apart for the performance of a special 
sort of work or physiological labor. This separation of 
parts for particular or special functions is called differentia- 
tion ; and the highest animals are those whose bodies are 
most differentiated, while the lowest are those whose bodies 
are least differentiated ; hence Aigh animals are specialized, 
and, on the other hand, Jow animals are simple. Thus dif- 
