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Gastrulation Sorts the cells of the blastula into 3 distinct layers Cells on 1 part of the blastula begin to burrow inwards forming the blastopore This burrowing produces an external lip—the dorsal lip of the blastopore—which becomes the organizer for the gastrula Cells on the surface of the blastula roll over the lip and move into the interior of the gastrula Cells migrate along the dorsal wall of the blastocoel and become organized into mesoderm and endoderm Within the endoderm the archenteron forms (will form the gut) The blastopore lip enlarges to form a complete circle which surrounds the yolk plug (will become the anus) Ectoderm forms the cells remaining on the surface Differentiation By the late gastrula stage a cells developmental fate is fixed Heterogeneous distribution of cytoplasmic contents leads to regional differences the zygote During cleavage cells receive different substances (mRNA, proteins, etc) Gene expression in and the fate of cells are influenced by the cytoplasmic contents and by interactions among cells Determination Progressive restriction of a cells developmental potential After a certain time, even though a cell has all the genes present in an organism, its fate cannot be changed by moving it into another region of the embryo Organogenesis Germ layers give rise to early organs First evidence is folds, splits, and condensation of cells in the layers First organs to develop in chordates are the neural tube and notochord (organizer for the vertebrae and muscles) Neural tube formation The dorsal mesoderm above the archenteron condenses to form the notochord The ectoderm above the notochord thickens to form the neural plate which sinks into the mesoderm and rolls up to form the brain and spinal cord The notochord elongates and stretches the embryo lengthwise The mesoderm cells that form the vertebrae condense around the notochord More mesoderm condenses along the sides of the notochord forming somites— blocks of tissue that will form the muscles Morphogenesis Specific changes in cell shape, position, and adhesion that shape the embryo Tissues are created by cells “crawling” around using amoeba-like movements and the extra-cellular matrix as a guide Cells can secrete substances to promote or inhibit other cells to migrate Cells can receive signals with surface receptor proteins Pattern formation Development of an animal’s spatial organization along 3 axes Proximal-distal Anterior-posterior Dorsal-ventral Cells can respond to chemical or physical signals to know their location relative to other cells within the embryo Fate of the germ layers Ectoderm: epidermis, epidermal glands, inner ears, eye lenses, brain and nervous system Mesoderm: muscles, skeleton, kidneys, sex organs, circulatory system Endoderm: digestive tract lining, liver, pancreas, lungs