The early cleavages of the zebrafish egg are incomplete, or meroblastic. The same is the case for the egg of the chicken, with its much larger region of yolk. Primitively, the vertebrate egg probably underwent complete, or holoblastic cleavage, such as is present in some primitive fishes, in most amphibians, and in amphioxus. In the Zebrafish flipbook MooV you can count off 10 or 11 of such synchronous divisions, until, rather abruptly, you will see that synchrony is lost. At the midblastula transition, cell division synchrony is lost.
Cell cycles begin to get longer.
Epiboly , defined as a spreading out, a thinning, of the collection of cells, is the first morphogenetic movement. The cells rearrange during this morphogenesis. Deeper cells in the pile intercalate between more shallow ones, i.e., along radii of the blastula. Epiboly continues for about six hours, and during this time the blastoderm continues to thin and comes to completely cover the yolk. The movement seems much like pulling a ski cap down over your head.