1. Changes in birefringence and light scattering of the sea-urchin egg membranes at fertilization, mitosis and cleavage are described and illustrated.

  2. There is a close similarity between the changes in the cortex at fertilization and at cleavage.

  3. The changes in the cortex at cleavage start at the poles of the cell, and only later spread to the equator.

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Throughout this paper we describe the sign of birefringence with respect to the tangent. A negative tangential birefringence is of course the same as a positive radial one.

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