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 we 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 trace the 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. 



237 



