In (A–E), animal/anterior to the top and vegetal/posterior to the bottom. Drawings are oriented with the animal/anterior pole to the top and the ventral side to the left. The black arrowheads indicate the position of the introvert-trunk boundary and its movement during introvert retraction. Schematic drawings of priapulid embryos at days 3, 4, 5, and 6 of development (A″–D″), respectively, placed in the general context of priapulid embryonic development. The former vegetal pole, where the blastopore opened, becomes the anus (D′). (D, D′, and F) After 6 days of development, the mouth shifts to an anterior terminal position, and an actin-positive bundle that corresponds to the gut lumen (central phallacidin spot in F) runs from the mouth to the anus.
The mouth is formed on the ventral side of the introvert independently from the site of gastrulation. Note that the introvert shows a higher number of cells and smaller cells than the trunk. At 7 points of this infolding, an ectodermal cell of the introvert region forms an actin-rich outgrowth (E) that will become an oral scalid. At day 5 (C and C′), an inward ectodermal infolding appears at the vegetal pole (white arrowheads in C and D), which marks the introvert-trunk boundary (magnified in E from the white rectangle in C). A high number of mitoses (white arrows) in the ectoderm is observed, in particular in the animal hemisphere. After the invagination of the endomesoderm (B and B′), the archenteron collapses (white arrowhead in B), and the blastopore (asterisk) narrows. Gastrulation (A and A′) occurs at the vegetal pole and involves the formation of a small archenteron and a conspicuous blastopore (asterisk). 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′).