Mouse embryo. Macrophotograph of a prepared specimen of a mouse embryo after around 16 days of development. It has been made translucent and its skeleton stained with Alizarin red dye. The head is at top. The backbone runs down right, with the forelimbs below the mouth and the hind limbs and tail at bottom. Mice breed rapidly, with a gestation period of a few weeks, producing a litter of 3-12 young that are born naked and blind.