EUGLENA, AMOEBA, AND PARAMECIUM 291 



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 the Protozoa, 

 states that Amoeba starts out as a tiny pseudopodiospore Avhich 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. 



Behavior 



All of the activities of an animal which come in response to in- 

 ternal or external stimuli make up the ''behavior." The activities 

 of the animal under discussion include the formation of pseudo- 

 podia, ingestion of food, locomotion, and others. Amoeba proteus 

 exhibits either positive or negative reactions to various stimuli. An 

 environmental change to which an animal reacts is known as a 

 stimulus, while the reaction of the animal is called the response. The 

 movements made by an animal in response to stimuli are called 

 tropisms. Amoeba exhibits all of the tropisms discussed in Chapter 

 XIV. To physical contact, it responds positively if the impact is 

 gentle ; otherwise the response is negative. It responds negatively to 

 strong light and finds its optimum in a moderately reduced light. 

 When some part of the body surface of this animal comes quietly 

 into contact with food, there is a characteristic response. This part 

 of the protoplasm stops flowing while other parts continue, thus form- 

 ing a pocket around the particle of food. The edges of the pocket 

 fold in, meet, and join so as to enclose the object. This attraction to 

 food is likely a positive chemotropism. Amoeba reacts negatively 

 to concentrated salt, cane sugar, acetic acid, and many other chemi- 

 cals which have been tried. Amoebae have an optimum tempera- 

 ture range between 15° and 25° C. Temperatures approaching the 



