DEVELOPMENT OF BIOLOGY 



465 



published in 1838 and 1839. Together these studies clearly showed 

 that all organisms are composed of units, or cells, which are at once 

 structural entities and the centers of physiological activities. And 

 further that the development of animals and plants consists in the 

 multiplication of an initial cell to form the multitude of different 

 kinds which constitute the body of the adult. (Figs. 7, 32, 302, 

 303.) 



Unquestionably the cell concept represents one of the greatest 

 generalizations in biology, and it only needed for its consummation 



Fig. 302. — Matthias Jacob Schleiden. 



the full realization that the viscid, jelly-like material which zoolo- 

 gists interpreted as tha true living matter of animals, and the 

 quite similar material which botanists considered the true living 

 part of plants are practically identical. This viewpoint was crystal- 

 lized in the early sixties by Schultze (1825-1874) of Germany in 

 the formulation of the protoplasm concept, and thenceforth not 

 only morphological elements — cells — but also the material of 

 which they are composed — protoplasm — were recognized as 

 fundamentally the same in all living beings. Indeed, the realiza- 

 tion of a common physical basis of life in both plants and ani- 

 mals — a common denominator to which all vital phenomena are 



