AMOEBA OF CLASS SARCODINA 



89 



cretion occurs only by diffusion through the cell surface. There is 

 likely some excretion by this means in all Amoebae. 



Growth. — If there is increase in the volume of a body, this is 

 spoken of as growth. In all living organisms growth is accom- 

 plished by addition to the protoplasm. If food is plentiful, more 

 material is added to the protoplasm than is used up in the oxidation 

 which produces active energy. In other words, growth occurs 

 when the rate of anabolism exceeds the rate of catabolism in the 

 organism. 



Reproduction and Life Cycle 



The life history of the niajiy-celled animals to be studied later 

 includes a series of changes from egg, through embryo state, to 

 adult. In Amoeba the cycle is likely only partly known, because it 



Nucleus 



Contractile 

 iracaole 



Fig. 35. 



-Diagram to show fission in amoeba. A, Beginning of the process; B, 

 fission nearing completion. (Drawn by Joanne Moore.) 



is difficult to maintain cultures in perfectly normal conditions for 

 sufficiently long periods to get this complete story. Ordinarily, the 

 animal grows when conditions are favorable until it attains a cer- 

 tain size; when this limit of size has been reached growth ceases. 

 Why does the cell cease to grow? Why should it not attain the 

 size of a man? Or why should a tree not continue to grow until it 

 reaches the sky, or a man take on the proportions of an elephant? 

 We have not been able to put our fingers on any one factor that 

 completely controls growth. We do know of certain relationships 

 that influence it. It will be recalled that all materials used by a 

 cell must pass through the cell membrane, and likewise all waste 

 substances must be discharged in a similar manner. Mathematics 

 states that the volume of a cell increases according to the cube of 



