76 THE STORY OF LIFE'S MECHANISM. 



cells of plants. In the early part of this century 

 observations upon various kinds of animals and 

 plant tissues multiplied, and many microscopists 

 independently announced 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 formulated, 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 com- 

 posed 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 

 glands are all made up of similar 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 between 

 animals and plants. This discovery was like 

 that of our supposed supramundane observer 

 when he first found the human being that 

 brought into connection the widely different 

 cities in the various parts of the world. 



Schwann and his immediate followers, while 

 recognizing that the bodies of animals and plants 

 were composed of cells, were at a loss to explain 

 how these cells arose. The belief held at first 

 was that there existed in the bodies of animals 

 and plants a structureless substance which formed 

 the basis out of which the cells develop, in some- 

 what the same way that crystals arise from a 



