The lens consists of a core, called the nucleus, surrounded by an outer layer called the cortex, coated with a thin layer of cells called the capsule. (ashgi.org)
When cells from most animals are grown in a culture dish, they divide until they form a single layer of cells covering the base of the dish. (scientificamerican.com)
The ball's outer layer of cells gives rise to the placenta. (sciencenews.org)