ABSTRACT
The sea-urchin egg can be deformed by external force, and on removing the force it tends to round up. Assuming that the inner protoplasm is liquid, such a return to sphericity must be due to a force working at or near the surface of the egg. The surface force, or the tension at the surface, as referred to by Harvey, has been measured by several methods (Vlès, 1926; Harvey, 1931; Cole, 1932; Cole & Michaelis, 1932; Sichel & Burton, 1936; Norris, 1939). Most of them were reviewed by Harvey & Danielli (1938). Among these workers, however, Cole alone provided information which has been accepted as a proof that the cortex is elastic.
The author is indebted to Prof. H. Utinomi of the Seto Marine Biological Laboratory for the identification of the species.
Strictly speaking, the constancy of the tension alone does not exclude a possibility that the cortex is elastic. Theory of elastic shell implies that, if the membrane is highly stretched, the increase of the tension per cross-section is nearly exactly counterbalanced by the diminution of the thickness, and it is actually the case for the thin-walled rubber balloon (Mitchison, personal communication). For the cortex of sea-urchin egg, however, its low degree of initial stretch (Mitchison & Swann, 1934b) will preclude such a possibility.
