Surface contraction waves in amphibian eggs

Circular waves of change in brightness, known as 'surface contraction waves' (SCW-1 and SCW-2), propagate over the animal surface of amphibian eggs at each cycle of cleavage. Movement of carbon particles attached to the egg surface indicated that SCW-1 involves expansion of the egg surface, whereas SCW-2 accompanies surface contraction. Stiffness of the cortex as measured by applying negative pressure through a micropipette increased concomitantly with the passage of SCW-2. Measurement of stiffness at two loci on the egg surface with two sets of pipettes confirmed the spatio-temporal coincidence of the wave of stiffness and SCW-2. The stiffness showed either no change or even a slight decrease on passage of SCW-1. Thus SCW-2 is a genuine wave of 'contraction', but SCW-1 can more properly be called a 'surface relaxation wave'.