A Xenopus (frog) blastula (pre-gastrulation embryo) will undergo predictable movements in gastrulation to form it's three tissue layers, the ectoderm, mesoderm, and endoderm. A "fate-map" (top left) shows which regions of the blastula will develop into specific parts of the larva after normal development. The blastula germ layers are stacked from animal pole to vegetal pole. The process of gastrulation moves them relative to each other and elongates them. During gastrulation, the ectoderm epibolizes over the other germ layers as the mesoderm turns under itself at the dorsal lip of the blastopore. In this way, the endoderm will end up lining the gut with the mesoderm overlying it and the ectoderm will cover the entire embryo and form the nervous system.
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gastrulation, xenopus, frog, fate map, epiboly, ectoderm, endoderm, mesoderm, evodevo