PROTOPLASM OF PROTOZOA 63 



The tension at the surface of Amoeba has been measured by E. N. 

 Harvey and Marsland (1932). They injected drops of paraffin oil or 

 oHve oil into A. proteus and A. duhia and then subjected them to centri- 

 fugal force in the microscope-centrifuge. Because of the bouyancy of the 

 oil, the organisms became stretched. The amount of distortion was photo- 

 graphed and, under certain assumptions, a value for the order of mag- 

 nitude of the surface forces was calculated. In this way these authors 

 found the tension at the surface of A. duhia to give values of one to three 

 dynes per centimeter. In this form they concluded that "there can be no 

 appreciable turgidity due to resisting surface layers." However, in A. 

 proteus it was impossible to pull even large oil droplets out of the or- 

 ganism by the highest centrifugal force available. For this reason no 

 tension at the surface of this organism could be calculated, but they esti- 

 mated it to be about thirty times that for A. duhia. The surface of A. pro- 

 teus was described as a firm, tough, external layer. 



Since the surface membrane may be of ultramicroscopic dimensions, 

 its physical properties are not easily determined, and for this reason 

 much of our knowledge is of a theoretical nature. It has been held that 

 the appearance of a new membrane at the surface of a torn bit of proto- 

 plasm is due to the accumulation at the surface of substances, chiefly 

 lipoid, which tend to lower surface tension. However, Heilbrunn thinks 

 of this process of new surface membrane formation as a "surface precipi- 

 tation reaction," comparable in many ways to the clotting of blood. He 

 has produced evidence to show that the presence of calcium is a pre- 

 requisite for the formation of new surface membranes. In addition, he 

 holds that any factors which cause a release of calcium from its protein 

 binding cause the "surface precipitation reaction" to take place within 

 the cell interior, giving to it a froth-like appearance. 



Whatever may be the exact mechanism of new membrane formation, 

 the fact that a time factor is involved in its production from the cyto- 

 plasm, that it assumes increased tension over that of the underlying 

 cytoplasm, that its consistency and durability depend both upon the en- 

 vironmental medium and the specific character of the protoplasm from 

 which it is formed, as well as its semipermeable properties, all point to 

 the surface membrane as being a definite, organized structure (Cham- 

 bers, 1924). 



Recent researches upon the optical properties of cell membranes other 



