FIGURE 21 Scanning electron micrograph of uncompacted (left)and com their behavior. They suddenly huddle together, maximizing their con pacted (right)8-cell mouse em tact with the other blastomeres and forming a compact ball of cells bryo.(Photographs courtesy of C. Ziomek (Figure 21). This tightly packed arrangement is stabilized by tight junc tions that form between the outside cells of the ball, sealing off the inside of the sphere(Figure 22). The cells within the sphere form gap