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 zoélogists 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. Le 
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- 
