THE WONDERS OF LIFE 



clearly their physiological significance on the one hand 

 and morphological on the other. We will therefore 

 consider the cell first, then the person (or sprout), and, 

 finally, the stock (or cornius). 



Ever since the middle of the nineteenth century the 

 cell theory has been generally and rightly considered 

 one of the most important theories in biology. Every 

 anatomical, histological, physiological, and ontogenetic 

 work must build on the idea of the cell as the elementary 

 organism. Nevertheless, we are still very far from 

 having a general and clear agreement as to this univer- 

 sal and fundamental idea. On the contrary, the ablest 

 biologists still difi;er considerably as to the nature of the 

 cell or the elementary individual, its relation to the 

 whole of the multicellular organism, and so on. This 

 divergence of views is partly due to the intricacy of 

 the phenomena we find in the life of the cell, and partly 

 to the many and extensive changes that have been made 

 in the meaning of the term in the course of its employ- 

 ment. Let us first cast a glance at the various stages 

 of its history. 



When in the last third of the seventeenth century a 



V number of scientists, especially Malpighi in Italy and 



^ Crew in England, used the microscope for the first time 



in the anatomic study of plant structure, they notjced a 



certain build of the tissue that closely resembled the 



honeycomb. The closely packed wax cells, filled with 



honey, of the hive, which show a hexagonal appearance 



in section, are like the wood cells that contain the sap in 



/" the plant. It was the great merit of Schleiden, the real 



founder of the cell theory, to prove that all the different 



tissues of plants are originally composed of such cells 



V(i838). Theodor Schwann soon afterwards proved the 

 same for the animal tissues; in 1839 he extended the 

 theory to the whole organic world. Both these scientists 

 regarded the cell as essentially a vesicle, the firm mem- 



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