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		form the mesoderm (muscles & bones) and endoderm (gut) of the embryo while the cells that remain form the ectoderm (skin and brain). 2. Axis formation (D/V and A/P) The dorsal-ventral axis is created during the formation of the epiblast; dorsal is the side towards the blastocyst cavity. When the primitive streak forms, it establishes the anterior-posterior axis, with the head at one end. 3. Formation of extraembryonic membranes a. amnion b. chorion c. allantois d. yolk sac Although there is no yolk to distort it, gastrulation in humans is just like in chicks. The sides fold down to seal the embryo. The cells around the periphery form the extra embryonic membranes. The amnion is a fluid-filled sac that protects the embryo. The chorion is the site of gas exchange in the placenta and the allantois which forms the connecting stalk between teh embryo and the placenta. Mammalian embryos even form a yolk sac, which of course no longer serves a function in nutrition, but retains the role of being the first site of blood formation. 4. Organogenesis After gastrulation and the estyablishment of the basic body plan, the major organ systems are formed. Some organs, such as the brain, are formed exclusively from descendants of one tissue type. However, organogenesis typically involves interactions beween multiple groups of cells, often from more than one germ layer, brought together by the movements of gastrulation. The sense organs are formed from thickened regions of the ectoderm, known as placodes, influenced by their proximity to the brain.