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 latter is 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. 



