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Dinitrophenol and several other uncouplers of oxidative phosphorylation cause a rapid and reversible dispersion of the in vivo mitotic apparatus as judged by the disappearance of its birefringence in annelid and clam eggs. Malonate will block cleavage at high concentration but does not disperse the MA. It, in fact, causes it to increase in size and birefringence to some extent. Since other work has shown that the ATP level in the egg is depressed by the inhibitors of oxidative phosphorylation while it is not depressed by malonate, we hypothesize that ATP level, or the processes which maintain it are responsible for the structural integrity of the metaphase, in vivo, MA.