32 
COMPARATIVE ZOOLOGY. 
the cell-contents. 13 The nucleus is generally attached to 
the inside of the membrane, and is the centre of activity. 
Cells vary greatly in size, but are generally invisible to 
the naked eye, ranging from to Trrinnr an inch in 
diameter. About 4000 of the smallest would be necessary 
to cover the dot of this letter i. The natural form of iso¬ 
lated cells is spherical; but when they crowd each other, 
as seen in bone, cartilage, and muscle, their outlines be¬ 
come angular, either hexagonal or irregular. 
Within the narrow boundary of a simple sphere, the 
cell-membrane, are exhibited all the essential phenomena 
of life — growth, development, and reproduction. The 
physiology of these minute organisms is of peculiar inter¬ 
est, since all animal structure is but the multiplication of 
the cell as a unit, and the whole life of an animal is that 
of the cells which compose it: in them and by them all 
its vital processes are carried on. 14 
The structure of a cell can be seen in blood-corpuscles, 
by diluting with a weak (i per cent.) solution of salt a 
drop of blood from a Frog, and placing it under the mi¬ 
croscope. (See Fig. 63.) 
2. Tissues.—There are organisms of the lowest grade 
(as Gregarina) which consist of a single cell, living for and 
by itself. In this case, the animal and cell are identical: 
the Gregarina has as much individuality as the Elephant. 
But all animals, save these unicellular beings, are mainly 
aggregations of cells: for the various parts of a body are 
not only separable by the knife into bones, muscles, nerves, 
etc., but these are susceptible of a finer analysis by the 
microscope, which shows that they arise from the devel¬ 
opment and union of cells. These cellular fabrics, called 
tissues, differ from one another both chemically and struct¬ 
urally, but agree in being permeable to liquids — a prop¬ 
erty which secures the flexibility of the organs so essential 
to animal life. Every part of the human body, for exam- 
