Fig. 1. Blastocoel formation. All images represent sections through the center of embryos. (A) 26-cell embryo indicating the apical (a), lateral (l) and basal (b) surfaces of a cell lining the blastocoel (arrowheads). The E daughters are indicated by asterisks. (B) 4-cell embryo showing a small basal separation (arrowhead); cell names are listed. (C) Electron micrograph of a 28-cell embryo where the E daughters (dashed outline) have partially ingressed. Prominent spaces between cells are indicated by arrowheads. (D,E) Membrane contacts at the same, high magnification showing examples of (D) a small space (arrow) between lateral surfaces and (E) a large space associated with basal surfaces; arrows in E illustrate close apposition of membranes around the space. (F) 16-cell partial embryo derived from an isolated AB blastomere; note the large central cavity (arrowhead). (G) 16-cell par-3 mutant embryo showing abnormally large separations (arrowheads) between lateral membranes. (H) Double embryo after fixation showing a cavity (arrowhead) in the center of AB descendants; a subset of the eight AB descendants present are visible and labeled 1 or 2 according to origin. The P1 descendants (MS, E, P3, and C) from each half of the double embryo are labeled. N, nucleus. Intact embryos (A-C,G) are 50 µm. Bar (for D,E),1 µm.