embryos

Every year, a panel of judges at London's Wellcome Collection of medical photographs selects the best of the year's acquisitions. This striking collection, reproduced here, represents the best medical images of the year.

The 19 images cover a wide variety of subjects and techniques, from the above picture of aspirin crystals to a picture of a seed taken with an electron microscope.

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Embryos of the red-eyed tree frog have developed an interesting strategy to survive on a patchy supply of oxygen. To permeate the normally oxygen-deficient eggs, oxygen must first pass through a strong outer membrane. But even though tiny hairs called cilia stir the fluid inside these quarter-inch-diameter eggs, most of the oxygen is near the eggs’ exposed surface.

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To create this image, which won the popular vote in the 2008 Nikon Small World contest, 22-year-old Tomás Pais de Azevedo, a graduate student in evolutionary and developmental biology at the University of Lisbon in Portugal, removed an eight-day-old, two-inch-long chicken embryo from its egg and stained it with a dye that binds to cartilage. The process took three days, after which he photographed the embryo through a stereo microscope. The dark-blue areas of the chick indicate where the cartilage will ultimately solidify into bone.

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