INTRODUCTORY. 



[CH. I. 



with mucin, and in adipose tissue, where they become charged 

 with fat. 



The term cell was first used by botanists ; in the popular sense 

 of the word a cell is a space surrounded by a wall, as the cell 

 of a prison, or the cell of a honey-comb. 

 In the vegetable cell there is a wall 

 made of the starch-like material called 

 cellulose, within this is the living 

 matter, and a number of large spaces 

 or vacuoles filled with a watery fluid. 

 The use of the term cell by botanists 

 was therefore completely justified. 



But the animal cell is different ; as 

 a rule, it has no cell-wall, and no 

 vacuoles. It is just a little naked 

 lump of living material. This living 

 material is jelly-like in consistency, 

 possessing the power of movement, and 

 the name protoplasm has been be- 

 stowed on it. 



Somewhere in the protoplasm of all 



cells, genei'ally near the middle in animal cells, is a roundish 

 structure of more solid consistency than the rest of the proto- 

 plasm, called the nucleus. 



An animal cell may therefore be defined as a mass of protoplasm 

 containing a nucleus. 



The simplest animals, like the amoebse, consist of one cell only ; 



Fig. 2. Animal cell consisting 

 of protoplasm containing a 

 nucleus. 



Fig. 3. Amoebse ; unicellular animals. 



Fig. 4. Cells of the yeast plant in pro- 

 cess of budding ; unicellular 

 plants. 



the simplest plants, like bacteria, toruke, etc., consist of one cell 

 only. 



Such organisms are called unicellular. In the progress of their 

 life history the cell divides into two ; and the two new cells 

 separate and become independent organisms, to repeat the process 

 later on. 



