34 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. 



Fully developed 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 sack called the 

 cell wall, generally very well defined, and secondly, within the 



FIG. 2. FIG. 3. 



FIG. 2. Diagram of animal cell (ovum). (Gegenbaur.)-o. Granular protoplasm, 

 fc. Nucleus, c. Nucleolus. 

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



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 proto- 

 plasm in the later stage of growth. The cell sap and other mat- 

 ters were found less commonly present, and appeared still later in 

 the lifetime of the vegetable cell ; hence it was concluded that 



