362 THE FORMS OF CELLS [ch. 



The film becomes a mere bimolecular, or even a monomolecular, 

 layer; and at last we may treat it as a simple "surface of discon- 

 tinuity." So long as the changes due to imperfect equihbrium are 

 taking place very slowly, we speak of the bubble as "at rest"; it is 

 then, as Willard Gibbs remarks, that the characters of a film are 

 most striking and most sharply defined*. 



So also, and surely not less than the soap-bubble, is every cell- 

 surface a complex affair. Face and interface have a molecular 

 orientation of their own, -depending both on the partition-membrane 

 and on the phases on either side. It is a variable orientation, 

 changing at short intervals of space and time; it coincides with 

 inconstant fields of force, electrical and other; it initiates, and 

 controls or catalyses, chemical reactions of great variety and 

 importance. In short we acknowledge and confess that, in sim- 

 pHfying the surface phenomena of the cell, for the time being and 

 for our purely morphological ends, we may be losing sight, or 

 making abstraction, of some of its most specific physical and 

 physiological characteristics. 



In the case of the naked protoplasmic cell, as the amoeboid phase 

 is emphatically a phase of freedom and activity, of unstable equi- 

 librium, of chemical and physiological change, so on the other hand 

 does the spherical form indicate a phase of stabihty, of inactivity, 

 of rest. In the one phase we see unequal surface-tensions manifested 

 in the creeping movements of the amoeboid body, in the rounding- 

 off of the ends of its pseudopodia, in the flowing out of its substance 

 over a particle of "food," and in the current-motions in the interior 

 of its mass ; till, in the alternate phase, when internal homogeneity 

 and equilibrium have been as far as possible attained and the 

 potential energy of the system is at a minimum, the cell assumes a 

 rounded or spherical form, passes into a state of "rest," and (for a 

 reason which we shall presently consider) becomes at the same time 

 encysted f. 



* On the equilibrium of heterogeneous substances, Collected Works, i, pp. 55-353; 

 Trans. Conn. Acad. 1876-78. 



f We still speak of the naked protoplasm of Amoeba; but short, and far short, 

 of "encystment," there is always a certain tendency towards adsorptive action, 

 leading to a surface-layer, or "plasma- membrane," still semi-fluid but less fluid than 

 before, and different from the protoplasm within ; it was one of the first and chief 

 things revealed by the new technique of "micro-dissection." Little is known of 



