In biology, Nucleus is a double-membrane limited space in the eukaryotic cell containing chromosomes and nucleoli. The double membrane is perforated by the nuclear pores that allow the metabolic and genetic connection with the cytoplasm. The outermost envelope is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus (e.g., as a result of fusion of several cells – syncytium – or by nuclear division without cell separation, coenocyte). Some protists have two types of nuclei (macro- and micronucleus), and in the arachniophyta, there is the remnant of an ancient nucleus from an engulfed alga (nucleomorph). Albeit the evolutionary origin of the nucleus remains unsolved, several authors suggest that the nucleocytoplasm is the result of an ancient symbiotic association between a bacterial cell and an archaeal cell.