Cell Theory
You scrape your knee on the sidewalk, and a small patch of skin comes off. A few days later, a fresh pink layer fills in the gap. Those new skin cells did not appear out of thin air, and your body did not make them from scratch like plastic parts in a factory. They came from the living cells around the edge of the scrape. Each of those cells pinched down the middle and split in two to make more.
Every living thing is made of cells, from garden weeds to people. A new cell only ever forms when an existing cell divides in two. Your own body started as one single cell, and every cell in you today came from that cell dividing over and over. The rule says that "Every cell comes from another cell." This is where the memory line stops being true. That rule holds for every cell alive today, but the very first cells on Earth could not follow it. More than three billion years ago, the first cells did not have older cells to come from, and science is still working out how non-living chemicals first formed a living cell.
Therefore
Whenever you read about a tissue growing or healing, look for how existing cells divide to do the work. Living tissue never forms on its own out of raw fluid.
Robert Remak and Rudolf Virchow, 1855
In the 1830s, many scientists believed that new cells formed directly out of body fluid. In Berlin, Robert Remak studied frog eggs and bird embryos under his microscope. By 1852, he proved that animal cells only multiply when an older cell pinches in two. Three years later, in 1855, Rudolf Virchow in Würzburg published an essay that popularized the Latin phrase omnis cellula e cellula. Virchow became famous for the idea, but he had borrowed Remak's findings on cell division without naming him.
A small reminder, on a notebook or a laptop lid, that your whole body grew from a single cell that split in two over and over.
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