Z projections of confocal stacks of embryos at 3, 4, 5, and 6 days of development stained with phallacidin (green) and propidium iodide (magenta). The mouth is now at an anterior terminal position ( Figure 2F), and the gut lumen runs from the mouth to the anus, which is formed by four ectodermal cells at the vegetal/posterior pole of the trunk ( Figures 2D′, 2F, and S2). Actin filaments in the introvert and in the trunk indicate the developing musculature of the embryo ( Figures 2D and 2F). After 6 days of development, the introvert-trunk boundary is more pronounced ( Figure 2D). The developing anus is identified by a strong actin-positive bundle at the vegetal pole ( Figure S2). At this point, on the ventral (see below) side of the introvert (animal hemisphere), a group of ectodermal cells ingresses and forms the mouth of the embryo ( Figure 2C). The animal-vegetal axis of the egg thus corresponds to the anterior-posterior axis of the larva and adult. Along the upper ectodermal layer of the equatorial indentation, we detect actin-positive cell outgrowths in seven cells ( Figure 2E) that will form the presumptive epidermal buccal tooth-like structures of the larval introvert (“scalids” Figure 1C). Vegetal ectodermal cells below the equator of the embryo invaginate and form an indentation ( Figure 2E) that divides the embryo into a larger animal and a smaller vegetal hemisphere, which correspond to the introvert and trunk of the larva, respectively ( Figure 2C). Five days after fertilization, the introvert-trunk boundary ( Figures 1B and 1C) is established. Three days after fertilization, the invaginated endomesodermal cells define a small archenteron that opens to the exterior through a blastopore in the vegetal pole ( Figures 2A and 2A′ ), which narrows by the end of gastrulation ( Figures 2B and 2B′).