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AP-Biology Essay The egg of the amphibian is fertilized externally and contains a large amount of yolk After the egg is fertilized there is a reorganization of the cytoplasm When the sperm penetrates the egg the pigment cap rotates toward the point of sperm penetration and a gray crescent appears on the side of the egg opposite the point of the sperm entry Since the yolk is present in larger amounts the egg divides unevenly forming larger cells in the vegetal hemisphere The cleavage forms a blastula but the amphibian blastocoel is small and off center The blastopore appears as a crescent shaped slit between the gray crescent and the vegetal hemisphere Gastrulation the process of inward movement in amphibians is an extensive series of coordinated cellular movements migration of single cells and sheets of cells and the folding of cell layers The initiation of Gastrulation starts when the blastopore is formed The cells at the dorsal lip of the blastopore change shape and move t o the interior and are replaced by cells that move to the surface The cells move over the lip When the cells get into the embryo they move away from the blastopore deeper inside The direction they move will become the anterior-posterior axis of the amphibian The moving cells form a wall of increasingly spacious archenteron and the blastocoel disappears The archenteron remains open to the outside at the blastopore the site where the anus will be Yolk-laden cells are visible within the boundaries of the blastopore forming a yolk plug Gastrulation forms the endoderm mesoderm and ectoderm The roof is endodermal cells that have been pushed and pulled into the interior by a sheet of mesodermal cells that lies above it along the axis of the embryo The sheet will become the notochord and the cells form it are called the chordamesoderm Other cells between the ectoderm and endoderm form the lateral plate mesoderm The neural ectoderm
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