38 BIOLOGY 



2. The discovery of protoplasm and the development of the 

 mechanical theory of life, 1861 to about 1885. 



3. The discoveries of the functions of the nucleus and its 

 relations to reproduction and heredity, from about 1880 to the 

 present. 



While these periods are not sharply marked off from each 

 other, they do represent different epochs in the development of 

 the conception of the nature of the cell. 



i. THE EARLY CONCEPTION OF THE CELL (1839-1861) 



The Formulation of the Cell Theory, 1839. It was not 

 definitely proved until about 1839 that the tissues of animals 

 and plants were composed of cells, although cells were first 

 described in 1665 by Robert Hooke. A microscopic study of a 

 piece of cork showed him that it was made up of large numbers 

 of minute compartments which reminded him of the cells of a 

 monastery. Hence he gave them the name of cells, which they 

 still bear. Miscellaneous observations followed at intervals in 

 the next two centuries. In 1833 Brown described the nucleus 

 as a constant part of the cell. In the years 1838 and 1839 two 

 Germans, Schwann and Schleiden, one studying animals and 

 the other, plants, advanced the theory that the tissues of all 

 animals and plants were made up of these independent units, 

 to which they still gave the name of cells. These observations 

 formulated the so-called cell doctrine. 



The Original Conception of the Cell. It was first supposed 

 that the cell wall was the most essential part of the cell in con- 

 trolling the processes of life and separating die contents of the 

 cell from the surrounding medium. This conception did not 

 last long, for it was soon seen that there were many cells that 

 did not have cell walls. In these early days the existence of a 

 nucleus was not realized as of much significance. 



The Origin of Cells. In the beginning it was supposed that 

 cells were like crystals and developed from a cytoblastema as 



