Mesosomes (otherwise known as chondrioids) are an intracellular compartment (organelle) present in bacteria and absent in eukaryotes. These are generally more prevelent within Gram-positive bacteria as opposed to Gram-negative. They are thought to be formed through invagination of the cell membrane. They are commonly found near the site of cell divison.
Although once thought to be equivalent to mitochondria, no sufficient evidence has arisen to suggest that they present any unique function within the cell. Other membrane functions have been attributed to mesosomes, as their origin is thought to arise from membrane perturbation thanks to electron microscopy studies.