The fibrous structure of the mitotic apparatus (MA) isolated from dividing sea urchin eggs undergoes no changes visible in phase contrast during extended storage, but the solubility of the MA rapidly decreases after isolation. Polarization microscopy shows that a decrease in the birefringence of the MA also occurs after isolation and is correlated with the loss of solubility. This loss of birefringence indicates that some structural change takes place during this period, and such a change was demonstrated by means of electron microscopy. The tubular filaments which form the spindle of the intracellular MA and of the freshly isolated MA were found to break down during storage to rows of dense granules, this loss of continuity presumably accounting for the loss of birefringence. The interrelations of the observed changes and the significance of these observations for investigations on the isolated MA are discussed.

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