![]() The three germs layers, shown below, are the endoderm, the ectoderm, and the mesoderm. Image credit: Modified from Khan Academy Whether blastospore develops into a mouth or an anus determines whether the organism is a protostome or a dueterostome. ![]() The invagination will continue until it reaches the other side of the embryo, creating a both an anus and a mouth. The gray crescent is a specific region in Xenopus frog embryos that directs movement of cells during gastrulation. The outer layer of cells moves toward the blastopore, the location on the embryo where these cells invaginate to form the three embryonic layers, the ectoderm, the mesoderm, and the endoderm. During gastrulation, the cells of the embryo move dramatically. These animals, which include jellyfish and comb jellies, have radial rather than bilateral symmetry and have far fewer tissue types than triploblasts due the lack of a mesoderm. In protostomes, the blastopore becomes the embryo’s mouth in deuterostomes, the blastopore becomes the embryo’s anus.ĭiploblasts (animals with only two germ layers) do not have mesodermal cells. Another group of cells move to completely surround the embryo, forming the ectoderm, and a third group of cells move into the locations in between the outer and inner layers of cells, to form the mesoderm. The endodermal cells continue through the interior of the embryo until they reach the other side, creating a continuous tract through the embryo this tract is the archenteron, or embryonic gut. In triploblasts (animals with three embryonic germ layers), one group of cells moves into the blastocoel, the interior of the embryo, through an invagination called the blastopore. The specific details of gastrulation are different in among different animal lineages, but the general process includes dramatic movement of cells both across and inside the embryo: Recall that in some species, the information specifying the body axes was already present during cleavage as a result of cytoplasmic determinants and/or yolk polarity, but the axes do not become visible until after gastrulation.
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