MICHEL DELARUE, ISM / SCIENCE PHOTO LIBRARY MICHEL DELARUE, ISM / SCIENCE PHOTO LIBRARY
Sea urchin embryo, gastrula stage, light micrograph. In the gastrula stage of embryonic development, the cells move to form distinct germ layers, tissues that will develop into inner and outer surfaces (endoderm and ectoderm) and internal organs (mesoderm). The sea urchin has long been used as a model organism in reproductive biology.
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