PROTOPLASMIC STRUCTURE 127 



a weak solution of acid, which kills the cells. If then 

 they are placed in a weakly hypotonic solution containing 

 dye, the latter immediately enters as soon as the cell 

 swells; i.e., the continuity of the plasma membrane is 

 then no longer automatically maintained (as in living 

 cells) when the membrane is stretched or ruptured. 



Presumably in living protoplasm both metabolic 

 and purely physical factors (adsorption) take part in 

 the formation of new surface-films. There is evidence 

 that the protoplasmic surface-films undergo a change 

 in their physical properties soon after they are formed; 

 this is well shown in the freshly cut surfaces of sea-urchin 

 eggs. Kiister found that the protoplasm of plant 

 cells could be broken into fragments by strong plas- 

 molysis; these fragments at first readily unite or cohere, 

 but later lose this property.^ In the coalescence of frag- 

 ments of inert inorganic material a similar behavior has 

 been observed; freshly formed surfaces reunite readily, 

 but not older surfaces ; apparently the progressive deposi- 

 tion or adsorption of foreign materials at the surfaces 

 alters their properties and prevents fusion.^ There is 

 also evidence that the formation of new surface-films 

 plays an essential part in the normal return of irritable 

 cells to the resting state after stimulation; during the 

 period of recovery of excitability (refractory period) the 

 cell-surface is apparently the seat of progressive changes 

 of this kind (see below under refractory period). 



Hober cites the case of myxomycetes possessing a 

 clear surface hyaloplasm and a granular interior ;3 when 



'Kiister, Ber. deutsch. hotan. Ges., XXVII (1909), 589. 



' For an account of these phenomena cf. Bancroft's Applied Colloid 

 Chemistry, chap, v, on coalescence. 



3 Hober, op. cit. (1914), p. 64. 



