HERVE CONGE, ISM / SCIENCE PHOTO LIBRARY HERVE CONGE, ISM / SCIENCE PHOTO LIBRARY
Sea urchin embryo. Light micrograph of a sea urchin embryo at the blastula stage. At this point the embryo is a hollow ball of cells. The centre of the embryo is known as the blastocoel. At left are the thick cells of the vegetal pole, which makes up the endoderm. The endoderm forms the digestive tract, glands and the lungs. At right is the animal pole, which makes up the ectoderm. It forms epidermis and nervous tissue. The cells moving into the centre of the embryo are mesenchyme cells and make up the mesoderm. The mesoderm forms the connective tissue, including blood vessels and cartilage. Magnification: x452 when printed at 10 centimetres wide.
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