WALLACE O. FENN 377 



gTc - gTp + cTp = ox < 



But equation (7) as well as equation (9) proves that a cell may spread 

 without spreading to infinity; indeed, any position is possible. Thus 

 in equation (7) if « = + 1 and «i = + 1, then h — 2, i.e., twice the 

 radius of the cell when spherical, and the cell will not spread at all 

 on G. If n = -{- \ and m = — 1 , then h = and the cell will spread 

 to infinity. If, however, to take an intermediate case, « = + 1 

 and m = 0, then h = V 2 = 1.26, which means that the cell will 

 take the position of a hemisphere, having the same volume as the 



M 



original sphere. At this point cos A = = and A = 90°. 



n 



When m = there is neither gain nor loss of energy when an area 

 of the interface between G and the plasma is replaced by an equal 

 area of the interface between G and the cell. The explanation of the 

 hemisphere as the equilibrium shape under these circumstances (when 

 m = 0) is that in this position the surface of the cell exposed to the 

 plasma is at a minimum. This brings out the significant fact 

 that the area of the exposed surface of a liquid sphere of diameter, 

 d, which is spreading to infinity on a flat surface, first decreases, 

 passes through a minimum when the apparent diameter (diameter 

 of the base) is \.26d, then increases until, at an apparent diameter of 

 l.S6d, it is again equal to the original surface area, and finally in- 

 creases to infinity. Exactly comparable changes in the surface area 

 of the cell occur during the ingestion of a small particle, except that 

 the final increase is limited by the size of the particle instead of by 

 infinity. This is clearly the reason why adhesiveness is such a fami- 

 liar property of blood cells. Vv^e have thus been led to a definition of 

 what we mean by adhesiveness or stickiness of cells. A cell which is 

 stuck to a slide is one that is incompletely spread out by forces of surface 

 tension. The energy necessary to detach the cell is stored up as sur- 

 face energy on the newly formed surfaces. If the cell tears, leaving 

 a layer of protoplasm still clinging on the slide, we have an exception 

 in which the energy expended is merely a measure of the cohesion of 

 the protoplasm (surface tension between protoplasm and protoplasm). 

 It is of course possible that the natural rigidity of a cell will prevent it 

 from spreading out on a solid surface so far and, therefore, from 



