EPOCHS IN BIOLOGICAL HISTORY 399 



study, and attention began gradually to shift from more or 

 less superficial details to cell organization. This culminated in 

 the classic investigations of two German biologists, the 

 botanist SCHLEIDEN (1804-1881) and the zoologist SCHWANN 

 (1810-1882), published in 1838 and 1839. Together these 

 studies clearly showed that all organisms are composed of 



FIG. 205. Matthias Jacob Schleiden. 



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 multi- 

 plication of an initial cell to form the multitude of different 

 kinds which constitute the adult. Unquestionably the cell 

 concept represents one of the greatest generalizations in 

 biology, and it only needed for its consummation the full 

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



