MEMBRANES AND CELL-PROCESSES 137 



might coexist side by side in such a system, as appears, for example, to 

 be the case in liver-cells; in this way a "chemical organization," dis- 

 tinct from and yet dependent upon a structural organization, becomes 

 possible. 



Haptogen membranes formed thus by deposition of proteins at 

 phase-boundaries may show considerable density and impermeability. 

 The protein in such surface-films may undergo an alteration resem- 

 bling coagulation, assuming a relatively resistant and insoluble form, 

 Thus Eamsden was able to coagulate protein solutions by prolonged 

 shaking, and Robertson obtained thin films of coagulated casein, gela- 

 tine and protamine at the surface of chloroform droplets. Solid films 

 of albumo.se, saponin, and other substances are formed at the free sur- 

 faces of their solutions — the readiness with which such solutions are 

 thrown into foams depends in fact on this condition. The condensed 

 and insoluble protein films formed on chloroform droplets are strikingly 

 similar in many respects to those visible at the surfaces of cells like sea- 

 urchin eggs, and which apparently correspond to the outer layer of the 

 true plasma-membranes. 



To come now to more directly biological considerations : what is the 

 nature, chemical and physical, of the surface-film of living cells ? There 

 are few direct chemical analyses bearing on this question. Liebermann 

 found the vitelline membrane of the hen's egg to consist largely of a 

 keratin-like albuminoid. There is good reason to believe that modified 

 proteins belonging to this class enter very generally into the composi- 

 tion of the surface-films of cells. The tendency to deposit horny or 

 albuminoid material at the cell-surfaces is in fact remarkably wide- 

 spread in animals. Cuticular and epidermal structures, to which chem- 

 ical resistance and impermeability are physiologically essential, consist 

 typically of proteins belonging to this class; such proteins have re- 

 cently been called " scleroproteins " on account of their frequent pres- 

 ence in skeletal or cuticular structures. They are also abundant in the 

 intercellular materials of bone, cartilage and connective tissue. The 

 surface-films of many cells apparently have this composition. Thus in 

 echinoderm eggs the characteristic fertilization-membranes, which Pro- 

 fessor Jacques Loeb has shown to arise by separation of a surface-film, 

 consist apparently of modified protein. They are at least non-lipoid in 

 character and are remarkably resistant to reagents, resembling in these 

 respects the protein films formed at the surface of chloroform drop- 

 lets. The fertilization-membrane, after separation from the cell, 

 proves however to be much more permeable than the true plasma-mem- 

 brane, or semi-permeable external layer of the unaltered egg, so that it 

 probably corresponds to only a portion — probably the outer layer — of 

 this membrane. The presence of protein in the plasma-membrane of , (j 

 sea-urchin eggs is also indicated by the fact that the cytolytic action of 



VOL. LXXXII.— 10. '~S 4 " x 



