CHAPTER XI 
THE CELL THEORY—SCHLEIDEN, SCHWANN, 
SCHULTZE 
THE recognition, in 1838, of the fact that all the various 
tissues of animals and plants are constructed on a similar plan 
was an important step in the rise of biology. It was progress 
along the line of microscopical observation. One can readily 
understand that the structural analysis of organisms could 
not be completed until their elementary parts had been dis- 
covered. When these units of structure were discovered 
they were called cells—from a misconception of their nature— 
and, although the misconception has long since been cor- 
rected, they still retain this historical but misleading name. 
The doctrine that all tissues of animals and plants are 
composed of aggregations of these units, and the derivatives 
from the same, is known as the cell-theory. It is a general- 
ization which unites all animals and plants on the broad plane 
of similitude of structure, and, when wve consider it in the 
light of its consequences, it stands out as one of the great 
scientific achievements of the nineteenth century. There is 
little danger of overestimating the importance of this doctrine 
as tending to unify the knowledge of living organisms. 
Vague Foreshadowings of the Cell-Theory.—In attempt- 
ing to tracethe growth of this idea, as based on actual observa- 
tions, we first encounter vague foreshadowings of it in the 
seventeenth and the eighteenth centuries. The cells were 
seen and sketched by many early observers, but were not 
understood. 
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