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First thin surface layer of skin appears covering the embryo. |
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< Somite
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SIZE: 4.0 - 6.0 mm The arches that form the face and neck are now becoming evident under the enlarging forebrain. By the time the neural tube is closed, both the eye and ear will have begun to form. At this stage, the brain and spinal cord together are the largest and most compact tissue of the embryo. A blood system continues to develop. Blood cells follow the surface of yolk sac where they originate, move along the central nervous system, and move in the chorionic villi, the maternal blood system. More than 30 pairs of somites will have formed by now. Somites represent differentiated cells made up of various precursor cells. In each somite, the area closest to the neural tube forms the epaxial dermamyotome region that produces epaxial myotome. The area farthest from the neural tube forms the hypaxial dermamyotome region that produces hypaxial myotome. The area in between these two regions is the central dermamyotome, which produces the dermis. The ventral-medial region of the somite forms sclerotome that migrates to become cartilage. In the epaxial region, back muscle is formed from epaxial myotome and rib is formed of sclerotome. In the hypaxial region, intercostal muscle and the lowest part of the rib region are formed from hypaxial myotome. The middle part of the rib region is formed from epaxial myotome despite the fact that this region is in the hypaxial muscle region. The digestive epithelium layer begins to differentiate into the future locations of the liver, lung, stomach and pancreas. Reference: Gilbert, Scott. 2000. Developmental Biology, 6th edition. Sinaur: Sunderland. |
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