Epiboly connected with cleavage in morula and early blastula stages of Xenopus laevis, a study using time-lapse photography.

Measurements were made of the external surface areas occupied by animal and vegetal blastomeres and their daughter cells at successive cleavage cycles in 15 embyros of Xenopus laevis. On the animal side, after each cleavage a general area increase (epiboly) occurs from cycle 4 (16-cell stage, stage 5) to cycle 10 (stage 8 1/2), while on the vegetal side there is a slight general area decrease after each cleavage from cycle 6 to cycle 10. The comparison between the external surface areas of individual animal blastomeres and those of their daughter cells, visible at the next cycle, shows a significantly larger increase after radial than after tangential cleavage, a difference that may be connected with the insertion of new membrane into the external surface at radial cleavage.

The direction of cleavage waves and the regional variation in the duration of cleavage cycles on the dorsal side of the Xenopus laevis blastula.

The animal and the dorsal side of five embryos of Xenopus laevis were studied in detail from the 7th to the 13th cleavage by means of time-lapse cinematography. At each cleavage the regionally ordered sequence of blastomere divisions is visible in the films as a "cleavage wave", propagating about three times slower in the dorsal than in the animal view. In the dorsal view the waves run in an animal-vegetal direction, initially with a left-to-right deviation and in later cleavages converging on the region of the future blastopore.

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.