Peroxisomes are ubiquitous organelles in eukaryotes that participate in the metabolism of fatty acis and other metabolites. Peroxisomes have enzymes that rid the cell of toxic peroxides. They have a single lipid bilayer membrane that separates their contents from the cytosol (the internal fluid of the cell) and contain membrane proteins critical for various functions, such as importing proteins into the organelles and aiding in proliferation. Like lysosomes, peroxisomes are part of the secretory pathway of a cell, but they are much more dynamic and can replicate by enlarging and then dividing. Peroxisomes were identified as cellular organelles by the Belgian cytologist Christian de Duve in 1967 after they had been first described in a Swedish PhD thesis a decade earlier.