STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Frog embryo. Light micrograph of a frog embryo at the blastula stage. This stage in the embryo's development is produced by the cleavage (cell division) of a fertilised ovum (egg cell). At this point the embryo is a hollow ball of cells surrounding a central fluid-filled cavity called the blastocoel. The outer layer of cells is known as the blastoderm. At bottom is the vegetal pole, which will develop into the endoderm. The endoderm forms the digestive tract, glands and the lungs. Magnification: x30 when printed at 10 centimetres wide.
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