258 BIOLOGY AND ITS MAKERS 
Definition.—The definition of Verworn, 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 series 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. 
