ORGANIZATION. 
33 
which secures the 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, etc. 
In plants, the cell generally 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 {blastema), which may be wa¬ 
tery, 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 ; i. e., the blastema 
is fluid. But in the solid tissues the cells coalesce, being 
simply connected, as in the epidermis, or united into fibres 
and tubes. 
In the lowest forms of life, and in all the higher animals 
in their earliest embryonic state, the cells of which they are 
composed are not transformed into differentiated tissues: 
definite tissues make their first appearance in the Sponges, 
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 the immature animals are more uniform in 
composition than the higher or adult forms. In the Verte¬ 
brates 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-ves¬ 
sels, gullet, stomach, intestines—in fact, every cavity and 
canal — is called epithelium. It is a very delicate skin, 
formed of flat or cylindrical cells, and in some parts (as in 
the wind-pipe of air-breathing animals, and along the gills 
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