The next series of events represents the true marvel of embryology. The tiny cluster of cells hanging from the walls of the cellular balloon, the inner cell mass, divides furiously and begins to form two layers of cells—the outer one called the ectoderm, and the inner called the endoderm. And about three weeks after conception, a third layer of cells invades the two layers and lodges itself between them, like a child squeezing into bed between her parents. It’s now the middle layer, called the mesoderm. This three-layered embryo—ectoderm, mesoderm, endoderm—is the basis of every organ in the
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