THE CELL AND PROTOPLASM. 



75 



THE CELL DOCTRINE. 



The cell doctrine is, in brief, the theory that 

 the bodies of animals and plants are built up 

 entirely of minute elementary units, more or 

 less independent of each other, and all capable 

 of growth and multiplication. This doctrine 

 is commonly regarded as being inaugurated in 

 1839 by Schwann. Long before this, however, 

 many microscopists had seen that the bodies of 

 plants are made up of elementary units. In 

 describing the bark of a tree in 1665, Robert 

 Hooke had stated that it was composed of little 

 boxes or cells, and regarded it as a sort of honey- 

 comb structure with its 

 cells filled with air. The 

 term cell quite aptly de- 

 scribes the compartments 

 of such a structure, as 

 can be seen by a glance 

 at Fig. 7, and this term 

 has been retained even 

 till to-day in spite of 

 the fact that its original 

 significance has entirely 

 disappeared. During the 

 last century not a few 

 naturalists observed and 

 described these little vesicles, always regarding 

 them as little spaces and never looking upon 

 them as having any significance in the activities 

 of plants. In one or two instances similar 

 bodies were noticed in animals, although no 

 connection was drawn between them and the 



FIG. 7. A bit of bark show- 

 ing cellular structure. 



