86 



THE STORY OF LIFE'S MECHANISM. 



FIG. 20. A muscle fibre. 



perties we are only concerned in the cell sub- 

 stance, the formed material 

 having nothing to do with 

 fundamental activities of 

 life, although it forms 

 largely the secondary ma- 

 chinery which we have 

 already studied. 



In all higher animals and 

 plants the life of the in- 

 dividual begins as a single 

 ovum or a single cell, and 

 as it grows the cells in- 

 crease rapidly until the 

 adult is formed out of 



The nuclei! are shown hundreds of millions of 



cells. As these cells become 

 numerous they cease, after a little, to be alike. 

 They assume different shapes which are adapted 

 to the different duties they are to perform. 

 Thus, those cells which are to form bone soon 

 become different from those which are to form 

 muscle, and those which are to form the blood 

 are quite unlike those which are to produce the 

 hairs. By means of such a differentiation there 

 arises a very complex mass of cells, with great 

 variety in shape and function. 



It should be noticed further that there are 

 some animals and plants in which the whole 

 animal is composed of a single cell. These 

 organisms are usually of extremely minute 

 size, and they comprise most of the so-called 

 animalculse which are found in water. In such 

 animals the different parts of the cell are modi- 



