French biologist Félix Dujardin theorized that the basis of all living creatures was the single cell. He would go onto observe within the cells of living organisms the same internal substance. His work further led to the development of cell theory.
Among his other discoveries, Dujardin disproved the theory that all animals have the same internal organs. He discovered that cells were not hollow but rather contained a substance that would later be termed "protoplasm." He also took single-celled organisms and grouped them together. He called them "Rhizopoda." Today this group is called protozoans.
In his life, Dujardin studied geology, optics, cryptology and botany. He also worked for a time as an engineer in hydraulics.