1. Measurements with the cell elastimeter on the stiffness of the cell membrane of fertilized sea-urchin eggs show the following general features. There is a sudden rise at fertilization, followed by a fall during the early sperm aster stage to the lowest value reached during development (a Young's modulus of about 0.58 x 104 dynes/cm.2). The stiffness rises slowly until metaphase, after which it rises rapidly to reach a maximum during late anaphase and early cleavage (6.81 x 104 dynes/cm.2). During the later stages of cleavage the stiffness falls again and reaches a value in the second interphase which is about twice as high as in the first interphase. Masurements on naked eggs in calcium-free sea water indicate that the slow rise in metaphase is due to the development of the hyaline layer.
2. Measurements on swollen and shrunken eggs at cleavage indicate that there is no interal pressure in the eggs at this stage, but similar experiments with eggs at the sperm aster stage yield anomalous results. Observations on the wrinkling point in shrunken eggs show that the maximum possible internal pressure is 19 dynes/cm.2 for sperm aster eggs and 500 dynes/cm.2 for cleaving eggs.
3. The bearing of these results on various theories of the mechanism of cleavage is briefly discussed. The rise in Young's modulus of the whole cell surface at cleavage argues against theories depending on the action of the spindle and asters, and against theories proposing a contractile ring in the surface. The rise is, however, what might be expected on the basis of the expanding membrane theory.