362 THE HISTORY AND SCOPE OF ZOOLOGY IX 



to a single focus. This was taken by Theodore Schwann 

 (1810-1881), who in 1839 published his epoch-making 

 cell-theory. Schwann was a pupil of Johann Miiller, 

 and there can be little doubt that the ideas of the 

 pupil are to be credited in some measure to the master. 

 Schwann took up the thread of microscopic investiga- 

 tion which had been sedulously pursued by a distinct 

 line of students since the days of Hook and Leeuwen- 

 hoek, and had resulted in a general doctrine among 

 botanists of the cellular structure of all the parts of 

 plants. Schwann showed not only that plants are 

 uniformly built up by these corpuscular units (of 

 which Eobert Browne in 1833 had described the 

 peculiar nucleated structure), but that all animal 

 tissues are also so built up. That, however, was not 

 Schwann's chief point. The cell-theory for which he 

 is famous is this, that the substance of the individual 

 cell is the seat of those chemical processes which seen 

 en masse we call life, and that the differences in the 

 properties of the different tissues and organs of animals 

 and plants depend on a difference in the chemical and 

 physical activity of the constituent cells, resulting in 

 a difference in the form of the cells and in a concomi- 

 tant difference of function. Schwann thus pointed to 

 the microscopic cell-unit as the thing to be studied in 

 order to arrive at a true knowledge of the processes 

 of life and the significance of form. In founding the 

 study of cell-substance (or protoplasm, as it was sub- 

 sequently called by Max Schultze in 1861, adopting 

 the name used by botanists for vegetable cell-contents) 

 Schwann united two lines of inquiry, viz. that of 



