Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a syncytium.
From the en.wikipedia.org
Externally, the skin has a thin cuticle covering the epidermis, which consists of a syncytium with no cell walls.
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The epidermis is either a syncytium or a single layer of cells, and is covered by a thick collagenouscuticle.
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The heart is effectively a syncytium, a meshwork of cardiac muscle cells interconnected by contiguous cytoplasmic bridges.
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The syncytium of cardiomyocytes in the heart is tethered within a matrix composed principally of type I fibrillar collagen.
From the nature.com
The syncytium is traversed by a series of branching tubules containing fluid and is controlled by a few wandering, amoeboidnuclei.
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Consequently cytoplasmic bridges assure connection between the clones of differentiating daughter cells to form a syncytium.
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In contrast, the fruit fly zygote first forms a sausage-shaped syncytium, which is still one cell but with many cell nuclei.
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To be deemed valid, this theory needs a demonstrable example and mechanism of generation of a multicellular organism from a pre-existing syncytium.
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More examples
A mass of cytoplasm containing several nuclei and enclosed in a membrane but no internal cell boundaries (as in muscle fibers)
In biology, a syncytium (also spelled syncitium, plural syncytia) is a large cell-like structure filled with cytoplasm containing many nuclei. Most cells in eukaryotic organisms have a single nucleus; syncytia are specialized forms used by various organisms.
A mass of cytoplasm containing many nuclei
Greek syn = with, and kytos = cell, hence a multinucleate mass of protoplasm, formed by the merging of cells.
A collection of cells with more than one nucleus.
A multinucleate cell. (Pl. syncytia.) (14)
A mass of protoplasm with multiple nuclei but without differentiation into separate cells.
A multinucleate tissue in which cell membranes don't completely separate the nuclei. In the Drosophila embryo, during cleavage, the blastoderm first consists of a syncytium in which many nuclei are lined up in the peripheral cytoplasm.
A mass or clump of cells that fuse together to form one "giant cell." In HIV infection syncytium formation can may lead to direct cell-to-cell infection. Strains of HIV are classified as either syncytium-inducing (SI) or non-syncytium-inducing (NSI). ...