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Baby:
Your baby is now about the size of a grain of rice. A line, called the “primitive streak,” is forming along the length of the back plate, and the embryo takes on a more oblong shape. As these cells take on specialized functions, they move toward the vicinity of the organs they will eventually develop into.
Some of these cells converge between the two already existing layers of the embryo. The embryo now has three layers, each with its own set of tasks. The first layer will develop into the respiratory and digestive systems. The second layer will create bones, muscles, tissue, sex organs, kidneys and the circulatory system. And the third layer will make hair, skin, the brain and the spine.