CHAPTER II. 



THE CELL AND ITS STRUCTURE. 



The substance of all living beings, -whether plants or 

 animals, is made up of cells. The name does not 

 very appropriately describe what is meant by it ; the 

 reason of which is on this wise r — The existence of 

 cells was first noticed in plants, and in plants each 

 cell has a strong wall; the result of that is, that when 

 the tissue is cut through, the contents inside the wall 

 being often partially destroyed, or removed, and in 

 any case chiefly of a fluid nature, the cells appear as 

 hollows inside the walls, properly to be termed cells. 

 Further investigation showed that the existence of the 

 walls was, so to speak, accidental, and that the soft 

 jelly-like contents of the cell were its essential part. 

 In some plant-structures, and in animals, the small 

 units of jelly were found to exist without those hard 

 walls. The wall consists of cellulose, a purely vege- 

 table ' product : it may be detected by treating the 

 organism with iodine. That substance, which affords 



' With the exception of the Aseidians, among animals, which 

 present a substance resembling cellulose in chemical com- 

 position (see p. 234) ; but it does not coat the cells, as in 

 plants, but coats the whole animal, forming a skin. 



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