The changes in refractive index of the cytoplasm and nuclear sap of the dividing spermatocytes of Locusta migratoria were measured by the refractometric technique first employed by Barer and Ross in 1952. It was found that, while certain restricted regions behaved slightly differently, the refractive index of the greater part of the cytoplasm fell during the first part of prophase from 1.354 in the resting cell to about 1.346, and rose again steadily after the beginning of metaphase until it approximated to its original value at the end of telophase. These results did not accord very closely with previous refractometric measurements, but showed a marked agreement with a previous record of changes in viscosity made by Carlson in 1946. It is suggested that this parallel behaviour is best explained by assuming that shifts in water distribution occur in the dividing cell, and that during prophase the cytoplasm and nuclear sap become correspondingly more aqueous as the chromosomes become denser.

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