Page:Popular Science Monthly Volume 82.djvu/141

Rh 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," distinct 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 resembling coagulation, assuming a relatively resistant and insoluble form, Thus Ramsden was able to coagulate protein solutions by prolonged shaking, and Robertson obtained thin films of coagulated casein, gelatine and protamine at the surface of chloroform droplets. Solid films of albumose, saponin, and other substances are formed at the free surfaces 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 composition of the surface-films of cells. The tendency to deposit horny or albuminoid material at the cell-surfaces is in fact remarkably widespread in animals. Cuticular and epidermal structures, to which chemical resistance and impermeability are physiologically essential, consist typically of proteins belonging to this class; such proteins have recently been called "scleroproteins" on account of their frequent presence 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 Professor 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 droplets. The fertilization-membrane, after separation from the cell, proves however to be much more permeable than the true plasma-membrane, 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, sea-urchin eggs is also indicated by the fact that the cytolytic action of