26 



MANUAL OF PHYSIOLOGY. 



den, who considered the cells to be characteristic of plant tissue. 

 A few years later Schwann showed that the animal tissues were 

 also made up of cells, though not so completely, and that they 

 owed their origin and development to cell elements. Thus origi- 

 nated the cellular theory, which, with some modification, is now 

 the basis of all physiological inquiry. 



The first idea which was conveyed by the term cell varied much 

 from that which we now accept as a proper definition of such an 

 organic unit. 



Vegetable cells being the first discovered were taken as the 

 type of all. The main characteristics of these may be briefly 

 summed up. Firstly, a membranous sac called the cell wall, gen- 



FIG. 2. 



FIG. 3. 



FIG. 2. Diagram of animal cell (ovum). (Gegenbauer.) a. Granular 

 protoplasm, b. Nucleus, c. Nucleolus. 



FIG. 3. Liver cell of man, containing fat globules (b) and biliary mat- 

 ters. (Cadiat.) 



erally very well defined, and secondly, within the cell wall various 

 cell contents. Among the more conspicuous portion of the latter 

 may be mentioned the (1) nucleus, which lies in a soft, clear, 

 jelly-like substance called protoplasm, and (2) certain cavities 

 called vacuoles, which are filled with a clear fluid or "cell sap." 

 Further investigation of the life history of cells, particularly in 

 the early stages of their development, showed that the cell wall, 

 which played so important a part in the original conception of a 

 cell, was not always present, but was formed by the protoplasm in 

 the later stage of growth. The cell sap and other matters were 

 found less commonly present, and appeared still later in the life- 

 time of the vegetable cell ; hence it was concluded that they were 



