LIVING PROTOPLASM; THE AMCEBA 33 



of all liA'ing matter. Assuming the fact that the above 

 are tj'pical cell structures we may then define a cell as 

 follows: A cell is a boxlike body having an external wall 

 that incloses a Uquid called protoplasm, and in which 

 floats a particle called a nucleus. Cells may vary in size, 

 shape, and in the thickness of their walls, and may con- 

 tain other substances beside the liquid protoplasm. Our 

 examination of the other tissues of the body will determine 

 whether this definition is complete (see pp. 39-51). Mean- 

 while let us examine this liquid protoplasm a little more 

 fully. 



Protoplasm. — Some one has said that " The study of 

 physiology is the study of protoplasm." The meaning of 

 this statement lies in the fact that the action of all living 

 tissues depends upon the action of this substance. If 

 that is the case, it is certainly worth while to learn what 

 we can of its properties. Here, however, a difficulty arises. 

 It is easy to obtain tissues whose cells are filled with 

 this substance, but when these tissues are removed from 

 the plant or animal body in which they belong, the proto- 

 plasm which they contain is killed and shows only a few 

 of the properties of the living substance. Fortunately, 

 however, there exists in nature a tiny animal form, only 

 about a hundredth of an inch in diameter, which is made 

 up almost entirely of protoplasm, uninclosed by any wall. 

 Furthermore, this little animal, called the amwba, can be 

 picked up from the surface of leaves in the still pools 

 where it lives and be transferred alive, in a medicine 

 dropper, to the slide of a microscope, where we can watch 

 all its actions (see Ex. XXIV.). 



Living protoplasm ; the amoeba. — The amoeba is com- 

 posed of liquid substance (protoplasm) of about the con- 



EIJDY. PHT3.— 3. 



