METABOLIC GRADIENTS IN AMOEBA 91 



The most reasonable explanation of this occurrence is, as 

 Rhumbler has pointed out in an excellent discussion ('05, pp. 

 29 ff.), heterogeneous tension in a gelatinized surface, these dif- 

 ferences in the tension being due, of course, to differences in the 

 degree of gelation. One may say then that contraction in the 

 amoeba is the result of differential gelation. I do not undertake 

 to say why protoplasm undergoing gelation should develop the 

 property of contractility. The fact remains that emulsoid solu- 

 tions in general when they pass into the gel state do develop this 

 property, and I am quite content to leave the explanation to the 

 colloid chemists. Mathews in his Physiological Chemistry (2nd 

 edition, '16, pp. 232-233) has given an explanation. 



I should therefore state emphatically that the property of 

 protoplasm which we call contractility is nothing more or less 

 than the gelation of an emulsoid colloidal solution ; or, as we like 

 to say in science, gelation is the ' cause' of contractility. 



An observation made by Gruber ('12, p. 326 ff.) is of interest as 

 further proof that the folding of the ectoplasm in contraction is 

 due to coagulation. The normal contracted amoeba is a spheri- 

 cal mass studded with short pseudopodia-like projections. If 

 now the temperature be raised (30°C. in Gruber's experiment), 

 these projections disappear and the surface is smooth, and ob- 

 viously highly fluid. On lowering the temperature, the surface 

 again becomes corrugated. I have also frequently observed that 

 when the cultural conditions become unfavorable, the amoebae 

 cease to move, and take on a spherical form with a smooth sur- 

 face. That such amoebae have become fluid is indicated by the 

 very marked Brownian movement of their granules, as well as by 

 the great increase in the size of the contractile vacuole (also noted 

 by Gruber at high temperature) and in the amount of fluid in the 

 food vacuoles. One may say that in the amoeba a smooth sur- 

 face indicates an alteration of the colloid state toward fluidity 

 while a corrugated and folded surface indicates alteration toward 

 coagulation. 



The liquefaction, as I have already pointed out, is caused by 

 chemical changes in the amoeba involving probably the setting 

 free of substances which produce liquefaction or the removal 



