Intracellular Calcium and Cell Cleavage in Early Embryos of Xenopus Laevis

There is growing evidence that cell cleavage involves contraction in the region of the furrow and relaxation above the most distant parts of the spindle (see references 15 and 16) . The contractile mechanism has been likened to that of muscle, mainly as a result of studies of glycerinated models of dividing cells (7, 9) and extraction of an actomyosin-like material (8, 11) . Muscular contraction is activated by calcium ions, and various workers have implicated the Ca ion as a regulator of cell cleavage (15, 16) . We now report experiments designed to test this hypothesis. The results show that Ca ions are essential for cleavage ; but, at this stage, it is not possible to decide whether or not changes in the level of ionized Ca regulate cleavage as opposed to cleavage merely having a requirement for some ionized calcium .