258 BIOLOGY AND ITS MAKERS 



Definition. — The definition of Venvorn, made in 1895, 

 may be combined with this diagram: A cell is "a body con- 

 sisting essentially of protoplasm in its general form, including 

 the unmodified cytoplasm, and the specialized nucleus and 

 centrosome; while as unessential accompaniments may be 

 enumerated: (1) the cell membrane, (2) starch grains, (3) 

 pigment granules, (4) oil globules, and (5) chlorophyll gran- 

 ules." No definition can include all variations, but the one 

 quoted is excellent in directing attention to the essentials — • 

 to protoplasm in its general form, and the modified proto- 

 plasmic parts as distinguished from the unessential accom- 

 paniments, as cell membrane and cell contents. 



The definition of Verworn was reached by a scries of 

 steps representing the historical advance of knowledge regard- 

 ing the cell. Schleiden and Schwann looked upon the cell 

 as a hollow chamber having a cell-wall which had been 

 formed around the nucleus; it was a great step when 

 Schultze defined the cell in terms of living substance as "a 

 globule of protoplasm surrounding a nucleus," and it is a 

 still deeper level of analysis which gives us a discriminating 

 definition like that of Verworn. 



When we are brought to realize that, in large part, the 

 questions that engage the mind of the biologist have their 

 basis in the study of cells, we are ready to appreciate the force 

 of the statement that the establishment of the cell-theory 

 was one of the great events of the nineteenth century, and, 

 further, that it stands second to no theory, with the single 

 exception of that of organic evolution, in advancing bio- 

 logical science. 



