ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



pseudopodia and took on the appearance of what was supposed to be a 

 quite distinct kind of Amoeba known as Amoeba radiosa (Fig. 5, B). 

 A slight rise in temperature is reacted to by increased activity of move- 

 ment but if the rise be continued up to about 30° C. the increased activity 

 is only temporary and is followed by gradual slowing and eventually 

 by death. Exposure to a temperature of 35" C. is followed rapidly by 

 retraction of the pseudopodia and death. Reduction of temperature 

 below the normal is followed by a slowing of the movement without any 

 other apparent ill effects. Exposure to intense light is followed by 

 rounding off and death. Even ordinary daylight produces a reaction. 

 If the Amoeba is in a shadow it has been observed to draw back when 



it comes to the edge of the 

 shadow, and it is only in 

 comparative darkness as a 

 rule that such events of 

 normal life as feeding or 

 fission take place. The 

 Amoeba shows interesting 

 reactions to electrical con- 

 ditions. If a very weak 

 current of electricity is 

 caused to pass through 

 water containing indi- 

 viduals of Amoeba Umax 

 they are observed to creep 

 towards the cathode or 

 negative pole. Sudden re- 

 versal of the current is 

 followed by reversal of the movement of the Amoebae.^ Exposure to 

 the emanations of Radium is soon followed by rounding off and death. 



An instructive experiment which has been made on Amoeba is the 

 division of the living creature into two portions (merotomy). When the 

 experiment is successfully carried out it is usually found that the smooth 

 slippery nucleus slips to one side of the cutting edge and passes completely 

 into one of th| two portions of the cytoplasm. The result of the experi- 

 ment then is the production of two Amoebas which may be almost 

 exactly alike as regards size but which differ in this respect that one 



Fig. 5. 



Reaction of Amoeba Umax to slight allcalinity of water. 

 A, before adding alkali (creeping type) ; B, after adding 

 alkali (floating type). 



^ Such movements of an organism or of parts of an organism in response to 

 specific stimuli are designated in teclinical language by special names ending in 

 -tropism or -taxis. Thus chemiotaxis (chemical stimulus), phoiotaxis or heliotropism 

 (light), galvanotaxis (electrical), etc. 



