CLEAVAGE PERIOD (0.7- 2.2 h)

Modified from: Kimmel et al., 1995. Developmental Dynamics 203:253-310. Copyright © 1995 Wiley-Liss, Inc. Reprinted only by permission of Wiley-Liss, a subsidiary of John Wiley & Sons, Inc.

After the first cleavage the cells, or blastomeres, divide at about 15 minute intervals (Fig. 4, Fig. 5). The cytoplasmic divisions are meroblastic; they only incompletely undercut the blastodisc, and the blastomeres, or a specific subset of them according to the stage (Kimmel and Law, 1985a), remain interconnected by cytoplasmic bridges. The six cleavages that comprise this period frequently occur at regular orientations (Fig. 6, Fig. 7) so that one can see how many blastomeres are present are by their arrangement; counting them is unneccessary.

One can be quite accurate about staging during this period, as well as the early part of the next one, by using Nomarski optics to subdivide each cell cycle. All the cells of the blastodisc procede through their cell cycles synchronously, or nearly so. Nuclei are present and visible during about the first half of each cycle, i.e., during interphase, and the nuclear shapes change systematically (see below, Fig. 10). The nuclei are globular during very early interphase, become spherical by late interphase, and then, as the cells enter mitosis, their nuclei take on an ellipsoidal shape shortly before disappearing during prophase. The long axis of the ellipsoid predicts the orientation of the following cleavage. Mitotic chromosomes, present during the other half of the cleavage cell cycle, are more difficult to visualize. Near the end of mitosis, and heralding the cytoplasmic division, the blastomeres become more rounded in shape.

Detailed description of cleaveage stages.