Prokaryotes are primitive organisms that lack a cell nucleus or any other membrane-bound organelles. Most prokaryotes are single-celled (although some have multicellular stages in their life-cycles), and they are divided into two main domains, bacteria and archaea. Eukaryotes, on the other hand, are organisms whose cells contain a nucleus and are organized into complex structures enclosed within membranes. Most living organisms (including all animals, plants, fungi and protists) are eukaryotes.