27-cell embryo, 410 minutes.
Confocal images (A) are shown in a z-projection (B) and an interpretive tracing (C). The 1abc1 cells have just divided and nuclei are in telophase; 1d1 is still in interphase. 3D is in prophase. The drawings in Figures 8–21 are from camera lucida tracings (see Material and Methods). Most embryos are seen approximately from the animal pole, with the secondary axis oriented more or less vertically. Those in Figures 17, 18 and 21 are seen from more or less dorsal aspects. All visible nuclei are labeled; note that the 1m2 nuclei are labeled simply with their quadrant letter (a,b,c,d) in order to conserve space. In general, ectoblast cells form a cap on the surface of the embryo, sitting atop the large yolky macromeres; in early embryos, all of the ectoblast nuclei thus are found within a short focal distance. As epiboly proceeds, the ectoblast cap spreads out over the macromeres until it completely encloses them; at all times, ectodermal nuclei are distributed more or less evenly throughout this superficial cell sheet. The mesoblast daughters of 4d meanwhile spread out underneath the ectoblast sheet; below this layer remain the nuclei of the 3ABC macromeres and their mitotically quiescent daughters, which also tend to be localized toward the embryo's surface. The embryo is about 160 microns in diameter.