Observations and measurements with an interference microscope on sea-urchin eggs have established the following points.

(1) The refractive index (R.I.) of the cytoplasm of the unfertilized egg is 1.375, which is equivalent to a concentration of solid matter of about 25 per cent. Very similar values were obtained from eggs 10 minutes after fertilization.

(2) There may be a small drop in the R.I. during the interphase before the first cleavage and during mitosis, but this is not greater than about 0.2 per cent., which is equivalent to a change in concentration of 2 per cent.

(3) The nucleus of the unfertilized egg has an R.I. of 1.360, equivalent to a concentration of 16 per cent.

(4) The R.I. of the mitotic figures is lower than that of the cytoplasm, by a ratio of 0.89 in metaphase. This is probably due to the absence of granules in the mitotic figures, since a similar ratio is obtained between the clear and the granular regions in centrifuged eggs.

(5) In metaphase, the chromosomes have the same R.I. as the spindle and are invisible. During the later stages of mitosis their R.I. drops, and they become visible.

(6) The dry thickness of the fertilization membrane is about 160 Å. There is evidence of a colloid in the perivitelline space at a concentration of about 0.07 per cent.

(7) The jelly has a concentration of less than 0.0055 per cent.

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