![]() ![]() As in avian development, the inner cell mass of placental mammals splits into an upper layer called the epiblast and a lower layer called the hypoblast (see Figure 38.8). In Concept 38.2 we discussed the development of the mammalian inner cell mass and the outer trophoblast. Mammals and reptiles, including birds, are all amniotes (i.e., they produce eggs that develop extraembryonic membranes), so it is not surprising that they share certain patterns of early development. Placental mammals retain the avian/reptilian gastrulation pattern, even though they lack yolk. The blastopore becomes the anus.įigure 38.11: Gastrulation in Birds Because their eggs contain a large yolk mass, reptile embryos-including those of birds-have a flattened blastodisc and display a pattern of gastrulation very different from that of amphibians. These cells will form much of the endoderm and mesoderm of the developing embryo. As these smaller cells reach the blastopore, they too move inward so that the inward migration occurs all around the blastopore. Because the small cells of the animal hemisphere continue to divide more rapidly than the larger cells of the vegetal hemisphere, these smaller cells grow over and surround the larger cells of the vegetal hemisphere. 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
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