32 COMPARATIVE ZOOLOGY. 
erally 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 =$5 to todo0 Of an inch in 
diameter. About 4000 of the smallest would be necessary 
to cover the dot of this letter. 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 circle, the 
cell-membrane, are exhibited all the essential phenomena 
of life— growth, development, and reproduction. The 
physiology of these minute organisms is of peculiar in- 
terest, 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.” 
The structure of the cell is well illustrated by an egg, 
in which the shell represents the outer membrane, the 
white the cell-contents, the yolk the nucleus, and the 
gerininal spot of the yolk the nucleolus. It is also seen 
in blood-corpuscles, by diluting with a weak solution of 
salt or sugar a drop of blood from a Frog, and placing it 
under the microscope. (See Figs. 62 and 158.) 
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 sd cell are identical: 
the Gregarina has as much individuality as the Elephant. 
But all animals, save these unicellular beings, are mainly 
ageregations 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 
development and union of cells. These cellular fabrics, 
called ¢isswes, differ from one another both chemically 
