THE LIVING SUBSTANCE. 



39 



tures to which is conceded the value of a membrane arise as a 

 simple or complex pellicular membrane.^ 



[33] In the pellicles of artificial froths, osmosis is controlled 

 by the passive physical conditions. In living foams, osmosis is 

 controlled also by changes in the pellicular substance, which 

 are correlated with a physiological function or power of the 

 substance (contraction). This is proven by the fact that exist- 

 ing physical conditions are in a moment set aside under such 

 diverse stimuli as are supplied by reagents, or even mere 

 pressure or shaking ; the visible phenomena in all cases coin- 

 ciding with those correlated with contractile function of the 

 substance in typically contractile areas. 



ALVEOLAR LAYERS. 



[34] Although in living protoplasm a true "alveolar layer" of 

 Biitschli may very often be found beneath the pellicles which 

 limit masses, and also those which surround inclusions, as the 

 contractile vacuole, these appearances are in most cases transi- 

 tory or transitional only, and are constantly disturbed by the 

 action of other forces. Excepting cuticular and other substance 

 structures which have organized contractile activity, — and 

 these may occur at any point in a mass, — so-called " alveolar 

 layers" among the Protozoa and among many Metazoan struc- 

 tures are subject to constant disturbance. 



(a) On the other hand, in dead or dying protoplasm, in some 

 peculiarly relaxed, inert, or partially atrophied areas and masses, 

 in short, wherever and whenever there exists a marked relaxa- 

 tion of the substance, alveolar layers are beautifully distinct, 

 and form at points in relation to the mass which also bear out 

 their physical origin. 



It is significant that in almost all preserved material, espe- 

 cially where the method has been rather slow and sufficiently 



1 Membrane (Century Dictionary definition) : A thin, pliable, expansive struc- 

 ture of the body ; an expansion of soft tissue, or part, in the form of a sheet or layer, 

 investing or lining some other structure, or connecting two or more structures. The 

 term is used in the widest sense with little or no reference to the kind of tissue 

 which may be involved, the membranous quality depending upon thinness and 

 pliability, not on texture or fabric. 



