At the same time, cell divisions continue to increase the number of cells. Small blastomeres of the animal pole then begin to roll like a sheet over the dorsal lip of the blastopore and push into the blastocoel. To initiate gastrulation, some cells in the gray crescent invaginate to form a slitlike blastopore ( FIGURE 38.10). In eggs such as those of amphibians, which contain a moderate amount of yolk, cleavage continues to produce many ever-smaller cells in the animal hemisphere and larger cells in the vegetal hemisphere. Mesenchyme cells assist with gastrulation and will also form the temporary larval skeleton. The archenteron, or “first gut,” remains connected to the exterior via the blastopore. Invagination to form endoderm in this non-yolky egg is illustrated. 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