THE CELL AND THE CELL THEORY 35 



zation concerning animals. It was largely due to the careful 

 work of the latter that the cell theory was widely accepted 

 throughout the scientific world. 



The cell theory gave a basis for a comparison between plants 

 and animals, as well as a new point of view in the study of the 

 tissues of multicellular organisms. A host of investigators have 

 entered the fields of histology and cytology; in fact, these are 

 still the favorite subjects for anatomists. 



The early investigators believed the cell wall to be the im- 

 portant part of the cell. Cohn, in 1847, observed that the proto- 

 plasm of certain lower plants left the wall at certain times and 

 swam about in the water. This discovery initiated the idea that 

 the wall is of minor importance as compared with the substance 

 contained within it. It was later ascertained that many cells 

 have no walls at all. 



Perhaps the foremost of recent workers was Max Schultze, 

 who recognized the cytoplasm and nucleus as the principal con- 

 stituents of all cells, and, in 1861, defined a cell as a mass of proto- 

 plasm containing a nucleus. He also stated that both the nucleus 

 and cytoplasm arise through the division of the corresponding 

 elements of a preexisting cell. At the present time, though we 

 retain the old designation " cell " for the units of protoplasm, 

 it must be constantly borne in mind that the name refers to the 

 living substance rather than to the wall, which may or may not 

 be present. 



A more fitting close for this chapter cannot be found than the 

 words of Professor E. B. Wilson, the foremost American investi- 

 gator of cellular phenomena. 



" During the half century that has elapsed since the enuncia- 

 tion of the cell-theory by Schleiden and Schwann, in 1838-1839, 

 it has become ever more clearly apparent that the key to all 

 ultimate biological problems must, in the last analysis, be sought 

 in the cell. It was the cell-theory that first brought the structure 

 of plants and animals under one point of view, by revealing their 

 common plan of organization. It was through the cell-theory 



