MOTOR RESPONSES 273 



tion. In A. mira, for example, the anterior portion consists of a thin 

 sheet of hyaline cytoplasm in close contact with the substratum (Hopkins, 

 1938). The surface tension at the water-substrate interface is probably 

 greater than the combined surface tensions at the cytoplasm-substrate and 

 cytoplasm-water interfaces. This would result in a spreading of the cyto- 

 plasm over the substrate (like oil over water) wherever the two are in 

 contact, i.e., at the anterior end of the organism. 



With reference to a marine amoeba, Pantin (1923-31) contends that 

 the cytoplasm at the anterior end swells and extracts water "jrom the 

 posterior protoplasm of the amoeba itself, and that this will cause a 

 stream from the posterior to the anterior end." Presumably he holds that 

 the cytoplasm shrinks there and gives it off. However, that would neces- 

 sitate absorption of water during gelation at the anterior end, and elimina- 

 tion during solation at the posterior end, which is contrary to what is 

 ordinarily observed in the process of gelation and solation of gels. 



Marsland and Brown (1936) suggest that the forward flow is due to 

 increase in volume during solation at the posterior end, and decrease 

 in volume during gelation at the anterior end. They give no direct evi- 

 dence in support of this suggestion, but simply say that "The magnitude 

 of these volume changes in relation to the observed rate of flow is prob- 

 lematical." 



Responses of Amoeba to light are therefore probably due to localized 

 changes in (1) the elastic strength of the plasmagel, (2) the rate of 

 transformation of plasmasol to plasmagel and vice versa, or (3) the 

 firmness, extent, or region of attachment to the substratum (Mast, 1923, 

 1926a, 1931a). 



Shock-reactions. — Engelmann (1879) long ago observed that if 

 strong light is flashed on an amoeba, movement stops suddenly, but that 

 if the intensity is gradually increased, movement continues. This response 

 therefore depends upon the rate of change of light intensity. Such re- 

 sponses are usually designated "Schreckbewegungen," or shock-reactions. 

 They are closely correlated with adaptation. The shock-reaction in 

 Amoeba produced by light varies greatly. It may consist merely of mo- 

 mentary retardation in streaming in a localized region in a pseudopod, of 

 total cessation throughout the entire animal with reversal in direction of 

 streaming after recovery, or of any one of an endless number of modifica- 



