Occurrence of Birefringence in the Fertilized Egg of the Sea Urchin

Abstract

In peiiminary experiments it was observed that, after fertilization, the eggs of Strongylocetrotus purpuratus showed, near the periphery, light and dark phases when rotated between crossed Nicois. The effect, however, was partially masked by the opaque granules of the cytoplasm. In order to eliminate this difficulty the unfertilized ova were centrifuged at approximately 60,000 gravities for 3 minutes, flotation of the eggs being insured by the addition of sufficient M/1 solution of sucrose to the sea water in the tubes. After centrifugation the eggs were transferred to sea water and nlicroscopic preparations made in the usual way. The eggs appeared elliptical in shape, with the nucleus at the centripetal end and the opaque granules at the opposite pole occupying about ⅓ of the egg. The remaining ⅔ of the egg was transparent and showed no birefringent effects with polarized light. Within 3 minutes after fertilization, however, when the membrane was forining and afterward, the clear cytoplasm gave evidence of birefringence so definite that the angle between light and dark phases could be measured. In all 35 measurements of the angle were made and these yield a mean value of 31°, with extremes of 24° and 37°, the intermediate values being coilsistently distributed on a probability curve. Now the optical section of the egg is something less than 72μ, and since such a minute quantity of polarizing molecules (e. g., sugar) in solution would not be sufficient to rotate the beam of polarized light, it is necessary to assume that the rotation is due to a regular grouping of molecules to form a multimolecular lattice with birefringent properties similar to those of a crystal, a starch grain, a filament of silk, a muscle fihril.