(With Two Text-figures) I. VOLUME AND SHAPE CHANGES DURING HAEMOLYSIS

WHEN the mammalian red cell is put in a hypotonic solution, it undergoes an increase in volume accompanied by changes in shape. If the solution is sufficiently hypo-tonic, it swells until it reaches its "critical area " substantially the same as the area of the original biconcave disk (Ponder, 1937; Castle & Daland, 1937). The cell then haemolyses, losing salts and haemoglobin; the tonicity inside becomes equal to that of the surrounding fluid, but the haemoglobin concentrations inside and outside do not come into equilibrium, an excess of pigment ("residual haemoglobin") re-maining inside. If the hypotonic system is restored to isotonicity by the addition of NaCl, "reversal of haemolysis " occurs. The almost transparent haemolysate be-comes opaque, and under certain circumstances the ghosts.can be centrifuged down at moderate-speeds. Looked at from an osmotic point of view, the volume which they might assume is indeterminate; ghosts completely permeable to salts, as well as to water, would not shrink at all, ghosts impermeable to salts would shrink to a small fraction of their volume, and ghosts permeable to water but slowly permeable to salts (as these are, Davson & Ponder, 1938) might take up any volume'betwee