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Amphibians typically live in water or on land during the different phases of their life cycles. An amphibian’s early development must take place in a moist environment, because the jelly envelope surrounding the developing embryo easily dries out. In the case of the frog, the fertilized eggs then develop into water-bound tadpoles. These swimming creatures are well adapted to their watery environment—they initially lack legs, they use gills to respire, and they are herbivores, eating the underwater flora available to them. However, as they transform into adults, their bodies prepare for a totally different lifestyle. Practically every organ of this animal transforms, or metamorphoses, as the tadpole prepares for life as a land-dwelling, carnivorous frog.
Developmental biologist Lewis Wolpert said about gastrulation, "It is not birth, marriage, or death, but gastrulation, which is truly the most important time of your life."
Although humans and frogs differ greatly in the structure of the blastula (called a blastocyst in humans) and in the process of gastrulation, in each case gastrulation sets the stage for the formation of the adult body. In gastrulation, the three embryonic tissue layers move into the positions where they begin to develop into the adult’s organs and tissues.
In the accompanying animation, we examined gastrulation in the frog. In this process, endodermal tissue migrates from the surface of the embryo to the interior. Ectodermal tissue extends around the embryo by the process of epiboly. Mesodermal tissue extends from a band of tissue to take its place between the endoderm and ectoderm. The primitive gut—the archenteron—forms while the blastocoel recedes. In frogs and other animals of the deuterostomate lineage, the blastopore eventually develops into the anus of the animal.
The following is a list of the adult tissues and organs that develop from the three embryonic tissue layers:
Ectoderm
Brain and nervous system; lens of the eye; inner ear; lining of mouth and nasal canal; epidermis of skin; hair and nails; sweat glands; oil glands; milk secretory glands
Mesoderm
Skeletal system (bones, cartilage, notochord); gonads; muscle; outer coverings of internal organs; dermis of skin; circulatory system (heart, blood vessels, blood cells); kidneys
Endoderm
Inner linings of gut; respiratory tract (including lungs); liver; pancreas; thyroid; urinary bladder
