VITAL PHENOMENA 149 



the mercury was often so great that it engulfed the crystal. This 

 experiment is crude, the movement being very rapid and the 

 crystal immediately thrown out after being ingested. 



In order to explain the food taking of the ameba, a still further 

 localization of surface tension changes must be assumed 1 . If we 

 assume that a substance diffuses from the food, which in very 

 low concentration causes a reduction in surface tension of the 

 ameba, but in higher concentration causes an increase in the 

 surface tension, we can explain this form of food taking. When 

 the ameba comes within the sphere of influence of the food its 

 surface tension is reduced on that side, and it advances or sends 

 a broad pseudopod toward the food. The center of the advancing 

 edge of the ameba comes so close to the food that its surface 

 tension is increased and its forward motion is stopped. On each 

 side of this central area, the protoplasm continues to advance 

 and finally forms a ring around the food. The upper and lower 

 edges of this ring constrict and thus enclose the food together 

 with some water in a hollow sphere of protoplasm. 



Quincke (1888 a) supposed the surface of protoplasm to be 

 of an oily nature, and hence soap should lower the surface ten- 

 sion. It was. found, however, that soap, no matter what the con- 

 centration, always increased the surface tension of the ameba, 

 when it had any effect at all ('McClendon, 191 1 c). Since no 

 phase boundary is known whose tension is increased by the pres- 

 ence of soap in either phase, some indirect action of the soap 

 must be looked for. 



Lippman showed that electrical polarization of a phase boun- 

 dary reduces the surface tension, and that reduction of this 

 polarization increases the surface tension. We have seen that 

 there is abundant evidence to show that the plasma membrane 

 of living cells in the resting condition is electrically polarized, 

 and that increase in permeability reduces this polarization. In- 

 crease in permeability, therefore, increases the surface tension 

 of cells. This is demonstrated by the rounding up of an ameba 

 when it is strongly stimulated. If this stimulation is accom- 

 panied by increase in permeability, as in the case of certain plant 

 cells, and apparently in the case of the muscle cell, we should 

 expect a reduction of the electrical polarization, increase in sur- 

 face tension, and an approach toward the spherical form. 



