ORGANIZATION. 33 
and structurally, but agree in being permeable to liquids 
—a property which secures that flexibility of the organs 
so essential to animal life. Every part of the human body, 
for example, is moist: even the hairs, nails, and cuticle 
contain water. , The contents as well as the shape of the 
cells are usually modified according to the tissue which 
they form: thus, we find cells containing earthy matter, 
iron, fat, mucus, ete. 
In plants, the cell always retains the characters of the 
cell; but in animals (after the embryonic period) the cell 
usually undergoes such modifications that the cellular 
form disappears. The cells are connected together or 
enveloped by an intercellular substance (6lastema), which 
may be watery, soft, and gelatinous, firmer and tenacious, 
still more solid and hyaline, or hard and opaque. In the 
fluids of the body, as the blood, the cells are separate ; but 
in the solid tissues they coalesce, being simply connected 
together, as in the epidermis, or united into fibres and 
tubes. 
In the lowest forms of life, and in all the higher ani- 
mals in their embryonic state, the cells of which they are 
composed are not transformed into tissues: definite tissues 
make their first appearance in the Polyps, and they differ 
from one another more and more widely as we ascend the 
scale of being. In other words, the bodies of the lower 
and immature animals are more uniform in composition 
than the higher, adult forms. In the latter only are all 
the following tissues found represented : 
(1) Epithelial Tissue—This is the simplest form of cellu- 
lar structure. It covers all the free surfaces of the body, 
internal and external, so that an animal may be said to be 
contained between the walls of a double bag. That which 
is internal, lining the mouth, windpipe, lungs, blood-vessels, 
gullet, stomach, intestines—in fact, every cavity and canal 
—is called epithelium. It is a very delicate skin, formed of 
3 
