After a time the minute colourless particles which are imbedded in the flowing protoplasm are drawn towards and unite with the aggregated masses; so that the protoplasm on the walls being now rendered quite transparent is no longer visible, though some is still present, and still flows, as may be inferred from the occasional transport of particles in the cell-sap.
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Notes from Wikiquote
(Detractors sometimes claim Darwin thought that the cell was an undifferentiated mass of protoplasm. Anyone reading this paper will realize that Darwin thought no such thing.)