Endocytosis is a process whereby extracellular matter is transported in bulk to the cell's interior. To accomplish this the cell extends portions of the plasma membrane which surround the mass to be ingested. As these cytoplasmic projections meet, they fuse. The endocytic vacuole thus formed pinches off from the plasma membrane's inner aspect from where it may be transported deeper into the cell. Alternatively such structures may form when a portion of the cell surface invaginates followed by plasma membrane fusion at the mouth of the invagination. In either case, matter which was formerly outside the cell becomes internalized through a complex process which requires a coordinated interaction of the plasma membrane with contractile elements in the cytoplasm. Usually, the ingested matter is subjected to digestive attack. This occurs when the ingested vacuole merges with lysosmes and the contents of each mix. Lysosomes are vesicular bodies containing both digestive enzymes which break down macromolecules, and enzymes that catalyze pathways which mediate killing of certain microbes.