4 GROWTH 



which is commonly called cytoplasm, and a more or less spheri- 

 cal, somewhat grayish part, called the nucleus. Together these 

 two parts comprise a cell. The amoeba lives in water and moves 

 from place to place by a sort of flowing process. As the amoeba 

 moves about in the water it flows around bits of plant or animal 

 material, which are its food. If the food is sufficient in amount 

 it grows larger, but not indefinitely. As the amoeba reaches a 

 more or less definitely limited maximum size it separates or 

 divides into two approximately equal parts, each with a nucleus 

 and cytoplasm, which flow away as independent individuals 

 capable of growing larger and dividing again. If the amoebae 

 stuck together as they grew instead of separating and proceed- 

 ing on their separate ways, a mass of jelly big enough to see with 

 the naked eye would develop in time. Some organisms are like 

 that. Those interesting creatures the slime molds, which may be 

 found flowing over the surface and in the crevices of decaying 

 wood or leaves, begin their lives as individuals much like the 

 amoeba in appearance, size, and structure. As the slime mold 

 grows the cytoplasm increases in amount and the nuclei divide. 

 However, the new nuclei and their surrounding cytoplasms do 

 not separate from one another and go their several ways, but re- 

 main together. Thus the slime mold gets larger and larger until 

 it may consist of a mass of protoplasm several square inches in 

 area. 



Most living things with which we are commonly acquainted 

 do not grow into mere masses of jelly as does the slime mold. 

 They have leaves and roots and stems or legs and heads and eyes. 

 This is something more than mere increase in size. This process 

 which results in most organisms having definite shapes and parts 

 we refer to as differentiation or development. In the amoeba 

 growth is almost entirely increase in size.* The growth of 



* Some creatures under certain conditions grow smaller. Thus a minute animal, 

 the flatworm, if furnished with food will increase in size. If food is withheld 

 it will grow smaller and smaller until it is a fraction only of its maximum size. 

 Supplied with food it will again grow larger and so by controlling the food sup- 



