CHAPTER IV. 



THE CELL 



CELLS were observed in plants more than two hundred years 

 ago by the English botanist Robert Hooke, who described cork 

 (a part of the bark of certain trees) as made up of " little boxes 

 or cells distinct from one another." The word cell was after- 

 wards employed by other observers to designate the minute 

 crowded cavities appearing in sections of wood and other vege- 

 tal tissues, and the term soon came into general use. 



The name itself shows that cells were at first regarded as cavi- 



O 



ties like the " cells" of a honeycomb, surrounded by solid walls. 

 Even as late as 1840 many biologists were inclined to regard the 

 walls as their essential part. But as biological knowledge ad- 

 vanced, it was shown that the walls might be wanting, and that 

 the active and essential part of the cell is the living protoplasm, 

 not the cavity in which it lies or the lifeless walls which surround 

 it. The word cell became therefore as inappropriate as it would 

 be if applied to the honey within the honeycomb. Neverthe- 

 less, by a curious conservatism, the term was and is retained to 

 designate these structures, whether occurring in masses as con- 

 stituent units of the plant or animal body, or leading independent 

 lives as "one-celled" organisms. 



It was first fullv established by Schleiden in 1839, that hio-her 



' * & 



plants are entirely built up of cells ; and about a year later Schwann 

 demonstrated that animals have a like composition. This irener- 

 alization is universally recognized as the " cell-theory" of Schlei- 

 den and Schwann, though it is no longer a theory, but simply a 

 statement of fact. This conception of the structure of higher 

 organisms was subsequently extended to their physiology, their 

 vital actions being conceived as the sum of the co-ordinated indi- 

 vidual actions (i.e., the lives) of the cells. For example, a muscle 

 is not only composed structurally of cells, but its function of 

 contraction (cf. p. 2f) is likewise the resultant of the contrac- 



