Duration of cleavage cycles and asymmetry in the direction of cleavage waves prior to gastrulation inXenopus laevis.

The animal and the vegetative side of 15 embryos ofXenopus laevis were studied from the 5th cleavage to gastrulation by means of time-lapse cinematography. The duration of cleavage cycles, defined for the embryo as a whole as the period between the earliest blastomere divisions of one cycle to those of the next, varies quite a lot between individual embryos, both with respect to synchronous and lengthened cycles. Cycle lengthening may start at either cycle 10, 11 or 12. Cycle 13 deviates from the individual rhythm, and moreover its duration is inversely correlated with the period elapsing from the beginning of this cycle to the onset of gastrulation which occurs in cycles 14 or 15. In each cleavage cycle, the regional sequence of first blastomere divisions is visible on films as a "cleavage wave" runming over the animal cap. The direction of the waves varies in different embryos during the synchronous period but begins to change from cycle 10 onwards, resulting in a similar direction in most embryos prior to gastrulation: from the ventral/left to the dorsal/right half. This change reflects an asymmetry in the lengthening of the cycles in the animal cap: more dorsally than ventrally, and more on the right than on the left. The possible significance of the results for the timing of gastrulation and for the pattern of the future embryo is discussed.