90 TEXTBOOK OF ZOOLOGY 



its diameter; while its surface increases only according to the 

 square of its diameter. In other words, the amount of material in 

 a growing cell increases approximately twice as fast as the plane 

 surface needed to surround it. It is logical, then, to assume that a 

 point may be reached when the surface area will not be sufficient 

 for the passage of necessary materials into and out of a cell. There 

 is, however, considerable variation in the size of cells; hence it 

 seems there must be other factors besides volume and surface rela- 

 tion in operation. Modified surface and difference in the rate of 

 metabolism certainly would be factors affecting the size of the 

 organism. When Amoeba reaches the limit of size, a division oc- 

 curs. Binary fission, by which two new individuals are produced, 

 has been definitely established, and some other methods of reproduc- 

 tion have been presented. Calkins, an authority on Protozoa, states 



New cells /Nuclear fragment 



Pig. 36. — Diagram to show amoeba encysted and undergoing the process of sporu- 



lation. (Drawn by Joanne Moore.) 



that Amoeba starts out as a tiny pseudopodiospore which has only 

 one pseudopodium. It then passes through a growth period and 

 increases in complexity until it reaches the full-grown condition. It 

 then divides by binary fission into two daughters. When each daugh- 

 ter has grown to nearly twice its original size, fission is repeated. 

 Environmental conditions and the variety of Amoebae determine the 

 number of times this phase is repeated. Occasionally the fission seems 

 to be an amitotic one. At the close of the fission phase, there is a 

 period of encystment and subsequent sporulation. During the en- 

 cystment the protoplasm undergoes several divisions to produce the 

 several pseudopodiospores which later break from the cyst as infant 

 Amoebae. It is felt that the complete details of the life cycle of many 

 common Sarcodina are not yet available. 



