Determinations of the permeability coefficient for water show that the osmotic behavior of amphibian eggs (ovarian and coelomic eggs from Rana temporaria) cannot be accounted for by this factor alone. Comparison of the cortical stiffness of coelomic eggs with vitelline membrane i n iso-and hypotonic solutions indicate that a mechanical tension develops under hypotonic conditions, strong enough to resist osmotic swelling.
In Ca++-free media preservation of this tension is interfered with, but the effect is observed only after exposure for some hours.
C a + + does not change the diffusion coefficient for water i n the egg cytoplasm, nor the permeability coefficient for water through the cell membrane. Neither is the Young's modulus for the vitelline membrane influenced. A slight effect of C a + + on the cortical stiffness of naked coelomic eggs was observed. In hypotonic Ca++-free media no resistance to swelling develops in these eggs, and they disintegrate after a rather short time.
To determine the permeability to water the rate of DzO -HzO exchange was measured with the Cartesian diver balance. The stiffness was determined with a cell elastimeter described by Mitchison and Swann, and the osmotic swelling was determined by measuring the egg diameter with a n eyepiece screw micrometer.