VI] THE FORMATION OF MEMBRANES 283 



While this brief sketch of the theory of membrane-formation 

 is cursory and inadequate, it is enough to shew that the physical 

 theory of adsorption tends in part to overturn, in part to simplify 

 enormously, the older histological descriptions. We can no longer 

 be content with such statements as that of Strasbiirger, that 

 membrane-formation in general is associated with the "activity 

 of the kinoplasm," or that of Harper that a certain spore-membrane 

 arises directly from the astral rays*. In short, we have easily 

 reached the general conclusion that the formation of a cell-wall 

 or cell-membrane is a chemico-physical phenomenon, which the 

 purely objective methods of the biological microscopist do not 

 suffice to interpret. 



If the process of adsorption, on which the formation of a 

 membrane depends, be itself dependent on the power of the 

 adsorbed substance to lower the surface tension, it is obvious that 

 adsorption can only take place when the surface tension already 

 present is greater than zero. It is for this reason that films or 

 threads of creeping protoplasm shew little tendency, or none, to 

 cover themselves with an encysting membrane ; and that it is 

 only when, in an altered phase, the protoplasm has developed 

 a positive surface tension, and has accordingly gathered itself up 

 into a more or less spherical body, that the tendency to form a 

 membrane is manifested, and the organism develops its "cyst" 

 or cell- wall. 



It is found that a rise of temperature greatly reduces the 

 adsorbability of a substance, and this doubtless comes, either in 

 part or whole, from the fact that a rise of temperature is itself 

 a cause of the lowering of surface tension. We may in all pro- 

 bability ascribe to this fact and to its converse, or at least associate 

 with it, such phenomena as the encystment of unicellular organisms 

 at the approach of winter, or the frequent formation of strong 

 shells or membranous capsules in "winter- eggs." 



Again, since a film or a froth (which is a system of films) can 

 only be maintained by virtue of a certain viscosity or rigidity of 



* Strasbiirger, Ueber Cytoplasmastrukturen, etc. Jahrb. f. luiss. Bot. xxx> 

 1897 ; R. A. Harper, Kerntheilung und freie Zellbildung im Ascus, ibid. ; of. 

 Wilson, The Cell in Devdo-pment, etc. pp. 53-55. 



