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Mouse embryo. Animated scanning electron micrograph (SEM) of a sagittal (vertical and central) section that has halved a mouse embryo. A variety of internal organs and tissues are visible. The backbone runs down the left, and ribs are seen at the front. From top to bottom the brain, nasal passages, thymus, heart, lungs, liver, pancreas, stomach and intestines are all visible. Mice breed rapidly, with a short gestation period of a few weeks, producing a litter of 3-12 young that are born naked and blind