From comparisons by electron microscopy (EM) of organelles isolated by differential centrifugation in sucrose density gradients, and in situ organelles of Xenopus cleaving eggs, we propose a sequence of vesicle transformations leading to the formation and completion of interblastomeric membranes: 1) Mitochondria located under the extremity of the early groove form bulges which detach to form free vesicles. These vesicles grow by progressive assimilation of the material contained in lipid droplets. Finally, they become large glycogen-containing vesicles, the immediate precursors of interblastomeric membranes, and become confluent with the plasma membrane at the upper border of the furrow (Fig. 1, region 1). 2) This process is accompanied by migration of Golgi coated vesicles towards the newly incorporated precursor membranes where they are inserted. At a later stage, smooth endoplasmic reticulum (ser) vesicles are inserted into the growing parallel interblastomeric membranes. These two processes would represent successive protein additions to the previously realized membrane lipidic phase. 3) Besides these events, other large vesicles, possibly formed by coalescence of ser vesicles, transport an abundance of material (precursors of the contractile ring of the furrow?) to the site of the groove in the cortical plasma membrane. They could also play a role in the induction of furrow initiation.