Insensitivity to cytochalasin B of surface contractions keyed to cleavage in the Xenopus egg.

In fertilized eggs of Xenopus laevis a marked flattening of the pigmented animal hemisphere has been observed. The flattening begins 15-20 minutes before the appearance of the cleavage furrow. As the furrow develops, the pigmented surface relaxes and rounds up. The initial appearance of the furrow is thus shown to be a combination of furrow deepening and rounding up of adjacent pigmented surfaces. It is demonstrated that the flattening is not caused by gravity or osmotic mechanisms and that internal pressure is increased during the flattening. The flattening is interpreted to be an isodiametric contraction of the pigmented surface. The contraction is not inhibited by injected cytochalasin B in sufficient concentrations to completely inhibit cleavage furrow formation. These results are discussed with respect to the presence of two surface contractile systems, distinguishable on the basis of their differing sensitivity to cytochalasin B.