The resistance to deformation of polymorphonuclear neutrophile leucocytes under the conditions of our observations has been shown to be on the average considerably less than the resistance to deformation of large mononuclear leucocytes. It is recognized of course that the viscosity of leucocytes, as of other cells, may be markedly influenced by osmotic conditions (17), by the reaction of the suspending medium (18, 19), by temperature, or by injury (20, 21). Although the conditions of our observations were quite different from those of the body, they were nevertheless closely similar to those of simultaneous phagocytosis experiments in which the cells functioned exceedingly well (3). Moreover E. R. and E. L. Clark (22) have noted that polymorphonuclear leucocytes in the tails of living tadpoles were more fluid than the macrophages. And Goss (23) in microdissecting human polymorphonuclear neutrophiles reports that they are more fluid than the clasmatocytes and monocytes studied by Chambers and Borquist (24). Other types of leucocytes have in our experience seemed to fall between the large mononuclear and the polymorphonuclear leucocytes in their average resistance to the interfacial tensions.

The leucocyte of each type studied is surrounded by an exceedingly delicate membrane. This membrane appears under the dark-field microscope as a pale, silvery line not distinguishable by inspection alone from a simple phase boundary between two immiscible liquids. That this is a membrane, however, and not a mere interface between immiscible phases, seems certain. In the first place the cell cytoplasm and the suspending medium are not immiscible. When the cell organization is broken down by the interfacial tension the greater part of the cell contents is immediately dissolved or dispersed. Goss (23) has noted that when the membrane is torn with a microdissection needle disintegration at once spreads over the membrane and the cytoplasm undergoes profound change. Moreover it is improbable that a simple phase boundary could exist in the presence of so much protein, lipoid, and other surface active materials as are present in protoplasm; the tendency of these substances to lower the free interfacial energy must necessarily tend to their adsorption in the interface until, if sufficient material is available at the interface, an adsorption film or membrane may be formed.

Kite (25), in a pioneer microdissection study, described the polymorphonuclear leucocyte as "naked" protoplasm. The contradiction between this statement and those just made is more apparent than real. For the capacity swiftly to form a limiting membrane between itself and other liquids is an attribute of "naked" protoplasm, as has been shown by the beautiful experiments of Chambers (20).

The present study of the wetting properties of leucocytes shows that their external membranes are hydrophilic, a character suggesting a surface in which proteins, probably bound water and salts (27), possibly the polar radicles of soaps or fatty acids, rather than non-polar lipoid groupings, are predominantly exposed. This makes it the more remarkable that a cell of such fluidity as for instance the polymorphonuclear leucocyte, composed largely of water and of water-soluble materials, should maintain its integrity in an aqueous medium with the aid of a membrane so delicate and so mobile. The mobility of the membrane, frequently extended in forming new pseudopodia or spreading over the surface of particles being ingested, must require constant entrance into and exit from the membrane of component materials, and their constant reorganization there. The limiting factors in the reformation of such a membrane would be the amounts of adsorbable materials available and their rates of movement up to the surface rather than the time required for orientation there, since the latter phenomenon is exceedingly rapid. Harkins (29), for instance has calculated that at a water-water vapor interface at 20°C., from the area occupied by one molecule of water, a molecule would jump out into the vapor and a vapor molecule would fall into this area of the surface 7,000,000 times in one second; the time of orientation of the water molecule he estimates to be of the order of 1/100,000,000 second or less.

The mammalian erythrocyte possesses a surface membrane capable of being folded and of withstanding tension in the interface. This has also been stretched by microdissection needles (21).

The surface of the erythrocyte, as evidenced by its wetting properties, is relatively hydrophobic, relatively non-polar in character, as compared with the leucocyte. Evidence indicating that the erythrocyte surface contains both lipoid and protein components has been summarized in earlier papers (8, 30). We have little to add here other than to point out that the wetting properties of the chicken erythrocyte surface are similar to those fully described for the mammal.

A serious source of error in certain isoelectric point determinations is discussed.

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