THE CELL THEORY 251 



Mohl, a botanist, observed a similar jelly-like substance in 

 plants, which he called plant schleim, and to which he attached 

 the name protoplasma. 



The scientific world was now in the position of recogniz- 

 ing living substance, which had been announced as sarcode 

 in lower animals, and as protoplasm in plants; but there 

 was as yet no clear indication that these two substances 

 were practically identical. Gradually there came stealing 

 into the minds of observers the suspicion that the sarcode of 

 the zoologists and the protoplasm of the botanists were one 

 and the same thing. This proposition was definitely main- 

 tained by Cohn in 1850, though with him it was mainly 

 theoretical, since his observations were not sufficiently ex- 

 tensive and accurate to support such a conclusion. 



Eleven years later, however, as the result of extended 

 researches, Max Schultze promulgated, in 1861, the proto- 

 plasm doctrine, to the effect that the units of organization 

 consist of little masses of protoplasm surrounding a nucleus, 

 and that this protoplasm, or living substance, is practically 

 identical in both plants and animals. 



The effect of this conclusion upon the cell-theory was 

 revolutionary. During the time protoplasm was being ob- 

 served the cell had likewise come under close scrutiny, and 

 naturalists had now an extensive collection of facts upon 

 which to found a theory. It has been shown that many 

 animal cells have no cell- wall, and the final conclusion was 

 inevitable that the essential part of a cell is the semifluid 

 living substance that resides within the cavity when a cell- 

 wall is present. Moreover, when the cell-wall is absent, the 

 protoplasm is the "cell." The position of the nucleus was 

 also determined to be within the living substance, and not, 

 as Schleiden had maintained, within the cell-wall. The 

 definition of Max Schultze, that a cell is a globule of proto- 

 plasm surrounding a nucleus, marks a new era in the cell- 



