The archenteron will eventually squeeze the blastocoel out of existence.įigure 38.9: Gastrulation in a Sea Urchin Embryo During gastrulation, cells move to new positions. The archenteron elongates, assisted by contractions of wandering cells called mesenchyme cells. The outer layer of cells is now ectoderm, and the wall of the archenteron is endoderm and future mesoderm. Recall that in deuterostomes, the blastopore becomes the anus and the mouth (the second opening) forms opposite it. The new space that forms is the archenteron, or “first gut.” The opening into the archenteron is the blastopore. One wall of the blastula (the vegetal pole) bulges inward, or invaginates, as if someone were poking a finger into a hollow ball. The process is easiest to understand in radially cleaving eggs with little yolk, such as the much-studied eggs of sea urchins ( FIGURE 38.9). Just as the amount of yolk affects cleavage, it also affects how gastrulation occurs. The resulting embryo is known as a gastrula. These layers are formed by the process of gastrulation. Triploblastic animals (see Concept 23.1) create all of their organs and tissues from three basic germ layers-ectoderm, endoderm, and mesoderm. These cell movements create a three-layered body plan and set the stage for development of the first organs. Gastrulation involves major movements of cells from the external surface to the interior of the embryo. Concept 38.3: Gastrulation Produces a Second, then a Third Germ Layer

Early development stages blastopore