[Modification by heterspecific cytoplasm injection, of the chronology of the first division of the Amphibia ovum].

Transfer of cytoplasm had been made between virgin eggs of two species, species with very different times between activation of the eggs and the first cleavage. The first furrow appears on an average of 6 hrs. after activation in Pleurodeles eggs and 2 hrs. 30 min. in Xenopus eggs. Slow cleaving P. waltlii cytoplasm introduced in quickly cleaving X. laevis eggs acted by delaying the formation of the first cleavage furrow, which appeared more than an hour after Xenopus controls. An opposite effect was obtained with Xenopus cytoplasm in Pleurodeles eggs. Cleavage furrows were obtained 2 hrs. before Pleurodeles controls. The cytoplasmic effect involved in the control of rhythms of cleavage could be related to a clock system.