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Abstract.A large increase in surface area must accompany formation of the amphibian embryo first cleavage furrow. The additional membrane for this areal expansion has been thought to be provided entirely from cytoplasmic stores during furrowing. We have radioiodinated surface proteins of fertilized, precleavage Xenopus laevis embryos and followed their redistribution during first cleavage by autoradiography. Near the end of first cleavage, membrane of the outer, pigmented surface of the embryo and a short band of membrane at the leading edge of the furrow displayed a high silver grain density, but the remainder of the furrow membrane was lightly labeled. The membrane of the cleavage furrow is thus mosaic in character; the membrane at the leading edge originates in part from the surface of the zygote, but most of the membrane lining the furrow walls is derived from a source inaccessible to surface radioiodination. The furrow membrane adjacent to the outer, pigmented surface consistently showed a very low silver grain density and was underlain by large membranous vesicles, suggesting that new membrane derived from cytoplasmic precursors is inserted primarily in this location, at least during the later phase of cleavage. Radioiodinated membrane proteins and surface-attached carbon particles, which lie in the path of the future furrow, contract toward the animal pole in the initial stages of cleavage while markers in other regions do not, We suggest that the domain of heavily labeled membrane at the leading edge of the definitive furrow contains the labeled elements that are gathered at the animal pole during the initial surface contraction and that they include membrane anchors for the underlying contractile ring of microfilaments.