210 ' THE FORMS OF CELLS [ch. 



seen likewise to "round itself off," that is not an effect of "vital 

 contractility," but (as Hofmeister shewed so long ago as 1867) 

 a simple consequence of surface tension ; and almost immediately 

 afterwards Engelmann* argued on the same lines, that the forces 

 which cause the contraction of protoplasm in general may "be 

 just the same as those which tend to make every non-spherical 

 drop of fluid become spherical ! " We are not concerned here with 

 the many theories and speculations which would connect the 

 phenomena of surface tension with contractility, muscular move- 

 ment or other special 'physiological functions, but we find ample 

 room to trace the operation of the same cause in producing, under 

 conditions of rest and equilibrium, certain definite and inevitable 

 forms of surface. 



It is however of great importance to observe that the living 

 cell is one of those cases where the phenomena of surface tension 

 are by no means limited to the ovter surface ; for within the 

 heterogeneous substance of the cell, between the protoplasm and 

 its nuclear and other contents, and in the alveolar network of the 

 cytoplasm itself (so far as that "alveolar structure" is actually 

 present in life), we have a multitude of interior surfaces ; and, 

 especially among plants, we may have a large, inner surface of 

 "interfacial" contact, where the protoplasm contains cavities 

 or "vacuoles" filled with a different and more fluid material, the 

 "cell-sap." Here we have a great field for the development of 

 surface tension phenomena : and so long ago as 1865, Nageli and 

 Schwendener shewed that the streaming currents of plant cells 

 might be very plausibly explained by this phenomenon. Even 

 ten years earlier, Weber had remarked upon the resemblance 

 between these protoplasmic streamings and the streamings to be 

 observed in certain inanimate drops, for which no cause but 

 surface tension could be assigned f. 



The case of amoeba, though it is an elementary case, is at the 



same time a complicated one. While it remains "amoeboid," it 



is never at rest or in equilibrium; it is always moving, from one 



to another of its protean changes of configuration ; its surface 



tension is constantly varying from point to point. Where the 



* Beitrage z. Physiologie d. Protoplasma, Pfluger''s Archiv, n, p. 307, 1869. 

 t Poggend. Annalen, xciv, pp. 447-459, 1855. Cf. Strethill Wright, Phil. 

 Mag. Feb. 1860. 



