THE CELL AND PROTOPLASM. 



6l 



to-day in spite of the fact that its original signifi- 

 cance has entirely disappeared. During the last 

 century not a few natural- 

 ists 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, al- 

 though no connection was 

 drawn between them and 

 the cells of plants. In the 



FIG. 7. A bit of bark show- 

 ing cellular structure. 



early part of this century observations upon va- 

 rious kinds of animals and plant tissues multi- 

 plied, and many microscopists independently an- 

 nounced the discovery of similar small corpuscular 

 bodies. Finally, in 1839, these observations were 

 combined together by Schwann into one general 

 theory. According to the cell doctrine then for- 

 mulated, the parts of all animals and plants are 

 either composed of cells or of material derived 

 from cells. The bark, the wood, the roots, the 

 leaves of plants are all composed of little vesicles 

 similar to those already described under the name 

 of cells. In animals the cellular structure is not 

 so easy to make out ; but here too the muscle, the 

 bone, the nerve, the gland are all made up of sim- 

 ilar vesicles or of material made from them. The 

 cells are of wonderfully different shapes and widely 

 different sizes, but in general structure they are 

 alike. These cells, thus found in animals and 

 plants alike, formed the first connecting link be- 

 tween animals and plants. This discovery was 



