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Bathybius haeckelii was a substance that British biologist Thomas Henry Huxley discovered and initially believed to be a form of primordial matter, a source of all organic life. He later admitted his mistake when it proved to be just the product of an inorganic chemical process (precipitation).
In 1868, Huxley studied an old sample of mud from the Atlantic seafloor taken in 1857. When he first examined it, he had found only protozoan cells and placed the sample into a jar of alcohol to preserve it. Now he noticed that the sample contained an albuminous slime that appeared to be criss-crossed with veins.
Huxley thought he had discovered a new organic substance and named it Bathybius haeckelii, in honor of German biologist Ernst Haeckel. Haeckel had theorized about Urschleim ("primordial slime"), <!--was 'monera', but I don't know if this is accurate-->a protoplasm from which all life had originated. Huxley thought Bathybius could be that protoplasm, a missing link (in modern terms) between inorganic matter and organic life.
Huxley published a description of Bathybius that year and also wrote to Haeckel to tell him about it. Haeckel was impressed and flattered and procured a sample for himself. In the next edition of his textbook, The History of Creation Haeckel suggested that the substance was constantly coming into being at the bottom of the sea, "monera" arising from non-living matter due to "physicochemical causes."
Sir Charles Wyville Thomson examined some samples in 1869 and regarded them as analogous to mycelium; "no trace of differentiation of organs", "an amorphous sheet of a protein compound, irritable to a low degree and capable of assimilating food... a diffused formless protoplasm."