The basis of modern ideas about the emergence of multicellular organisms is the phagocytella hypothesis developed by I.I. Mechnikov.
In accordance with this hypothesis, multicellular organisms evolved from colonial flagellar protozoa. Among the cells of such a colony there were motive cells equipped with flagella, and nourishing – phagocytic prey and carrying it deep into the colony. Subsequently, e3 of the outer layer of cells, which performed integumentary and motor functions, formed an ectoderm – integumentary tissue, and of the cells involved in digestion – endoderm. The part of the cells that specialized in the function of reproduction became germ cells. Confirmation of this hypothesis is the existence of a phagocytellike organism – Trichoplax.
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