112 



THE RISE OF ANIMAL LIFE 



Fig. 7-4. Amoeba undergoing binary fission. 



parts, a process known as binary fission 

 (Fig. 7-4). After the cell has grown to a 

 certain size, it rounds up into a ball. The 

 nucleus divides first, then the entire cell 

 cleaves into two parts which are usually 

 spoken of as "daughter cells" (they are 

 called daughter cells perhaps because they 

 give rise to other cells; "son cells" would 

 be unable to do this). The amoeba seems 

 to have to reach a certain size before di- 

 viding; for if a small piece of cytoplasm is 

 cut off periodically as the amoeba grows, 

 never allowing the animal to reach the 

 proper size, it will not divide. 



Amoeba is "immortal," as was pointed out 

 in an earlier section (p. 83). If death 

 occurs, it comes only through accident. Oc- 

 casionally its watery environment may dry 

 up, leaving it to desiccate and die if no 

 provisions were made for it to exist through 

 periods of adverse conditions. When such 

 a times comes, the amoeba (there seems 

 to be some question about A. proteiis form- 

 ing a cyst although other amoebas do) 

 secretes an impervious outer covering 

 called a cyst (Fig. 5-2), which allows life 

 to continue at a very low ebb until it is 

 once more submerged in water. The cyst 

 then splits open and the amoeba resumes 

 its active life. This provision has made it 

 possible for the animal to survive long peri- 

 ods of unfavorable conditions and has been 

 an important factor in its survival. 



Behavior 



Another innate quality of amoeba which 

 makes possible its survival is its responsive- 

 ness to changes in its external world. Its 

 response is usually such as to protect itself 

 from harm or to lead it to a rich food area. 

 This is spoken of as its behavior. Although 

 gradual changes in intensity of light elicit 

 very little or no response, intense, sudden 

 light causes it to send out pseudopods in 

 such a way as to withdraw and move away 

 from the light source (Fig. 7-5). Likewise, 

 if a concentrated salt solution is placed near 

 the amoeba, a definite response is noted. 



