EDELMANN / SCIENCE PHOTO LIBRARY. EDELMANN / SCIENCE PHOTO LIBRARY.
The human embryo after 3-4 weeks, showing the head region (right) & the developing spinal cord (known as the notochord) curving from the head region the left. This structure (which arises from mesoderm tissue) grows forwards & backwards divides the embryo into left & right halves; the first evidence of polarity. Future examples are the manners in which muscles & tissues grow in opposite directions away from the axis of the spinal cord.
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