276 THE FORMS OF CELLS [ch. v 



presence, among the multitudinous other dift'erentiations of their 

 protoplasmic substance, of such more or less elastic fibrillae, 

 which play as it were the part of a microscopic skeleton*. 



But these cases which we have just dealt with, lead us to 

 another consideration. In a semi-permeable membrane, through 

 which water passes freely in and out, the conditions of a liquid 

 surface are greatly modified ; and, in the ideal or ultimate case, 

 there is neither surface nor surface tension at all. And this would 

 lead us somewhat to reconsider our position, and to enquire 

 whether the true surface tension of a liquid film is actually 

 responsible for all that we have ascribed to it, or whether certain 

 of the phenomena which we have assigned to that cause may not 

 in part be due to the contractility of definite and elastic membranes. 

 But to investigate this question, in particular cases, is rather for 

 the physiologist : and the morphologist may go on his way, 

 paying little heed to what is no doubt a difficulty. In surface 

 tension we have the production of a film with the properties of an 

 elastic membrane, and with the special peculiarity that contraction 

 continues with the same energy however far the process may have 

 already gor^e ; while the ordinary elastic membrane contracts to 

 a certain extent, and contracts no more. But within wide limits 

 the essential phenomena are the same in both cases. Our 

 fundamental equations apply to both cases alike. And accord- 

 ingly, so long as our purpose is mofphological, so long as what we 

 seek to explain is regularity and definiteness of form, it matters 

 little if we should happen, here or there, to confuse surface tension 

 with elasticity, the contractile forces manifested at a liquid 

 surface with those which come into play at the complex internal 

 surfaces of an elastic solid. 



* As Bethe points out (Zellgestalt, Plateausche Fliissigkeitsfigur unci Neuro- 

 fibrille, Anat. Anz. XL. p. 209, 1911), the spiral fibres of which Koltzoff speaks must 

 lie in the surface, and not within the substance, of the cell whose conformation is 

 affected by them. 



