20 ANATOMY OF THE RABBIT. 
Considered collectively, these functions are not so well illustrated 
in the higher or multicellular organisms, in which particular 
functions are assigned to particular cells, as in the lower unicellular 
organisms, in which all functions are discharged by a single cell. 
In simple or protozoan animals the protoplasm is seen to be capable 
of ingesting food-materials, of discharging waste, of changing its 
form, and of reacting in one way or another to stimuli arising out- 
side of the body. Moreover, the protozoan cell is capable of giving 
rise to new cells by division of its substance into two parts, which 
process originates in the nucleus, and is associated at some stage 
usually at least, with union or conjugation of parent cells. 
All the cells of the body of a multicellular organism are products 
of a single cell, the fertilized egg, but the larter .s a product of 
fusion of two primary elements, the spermatozoon of the male 
parent and the ovum of the female. The fertilized egg does not 
exhibit the functions of a one-celled body, but possesses the poten- 
tial of these functions, and the latter appear, to a large extent 
individually, in the differentiation of its division-products into 
specialized tissue-elements. 
It is in this way that the body of a multicellular animal must be 
founded with reference to the same elementary functions of life 
as those appearing in one-celled organisms. But the repeated 
division of the fertilized egg, in development toward the adult 
condition, gives rise by division of labor to a great variety of 
cells, each kind of which may be regarded as representing a minor 
aspect of some major function. 
The Tissues. 
The primary tissues of the body are of four kinds—epithelial, 
connective, muscular, and nervous. To these—the fixed .- 
tissues—are to be added the fluid substances, blood and lymph, in 
which the cell elements, the red and white corpuscles, or in the latter 
case the white elements alone, are suspended in a fluid medium. 
The following survey of the principal features of the tissues will 
serve to make clear the extent to which the gross appearance of organs 
depends upon tissue composition, though the account itself is in 
no way intended as a guide to the microscopic structure of the rabbit, 
which is more properly part of the subject matter of histology. 
