STRUCTURAL CHARACTERS OF ANIMAL ORGANISMS. 39 



also occur, having various duties to perform. They are mainly 

 found in the adult in the blood and lymph, and scattered through- 

 out the tissues, and are without a cell wall, and have no special 

 contents to mark their function. 



Among the differentiated cells we find many special characters, 

 adapting them to certain special duties, for all these cells are 

 modified from the original type and applied to the performance 

 of some special function. 



Space prevents even a short enumeration of the varieties of 

 cells met with in the tissues of plants, where they carry on all 

 the active functions of the organism as well as form the firm sup- 

 porting structures. 



The differentiation of a cell is accomplished by its own proto- 

 plasm, which forms new structural parts and sometimes seems to 

 diminish in quantity until finally an element is produced in which 

 there is no longer any protoplasm recognizable. 



We find then matured and differentiated cells which vary 



1. In shape, being spherical, flattened, fusiform, stellate, etc. 



2. In size, becoming smaller or larger. 



3. In their mode of connection, becoming attached in one 



way or another to neighboring cells or structures. 



Cells may also be classified according to their function, e. g., 

 Glandular, Nervous, etc., and the greater portion of the following 

 pages will be devoted to the functions of these various forms of 

 cells. 



So long as a cell remains in its indifferent stages it possesses the 

 properties of ordinary protoplasm only ; but by its further devel- 

 opment it acquires special properties not common to all protoplasm. 

 These properties may or may not be accompanied by structural 

 change. Thus the protoplasm of a gland cell differs in little from 

 that of any other cell except in the capabilities of its nutritive 

 changes and its chemical products ; while on the other hand, those 

 epithelial cells which form the outer layer of the skin lose com- 

 pletely their protoplasmic characters and are profoundly modified 

 in structure. 



Modifications of Original Cell Tissues, In the pre- 

 ceding pages the special characters of a single cell have been dwelt 



