The nuclear envelope is a double-layered membrane that encloses the contents of the nucleus during most of the cell's lifecycle. The outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum (ER), and like that structure, features numerous ribosomes attached to the surface. The outer membrane is also continuous with the inner nuclear membrane since the two layers are fused together at numerous tiny holes called nuclear pores that perforate the nuclear envelope. These pores regulate the passage of molecules between the nucleus and cytoplasm, permitting some to pass through the membrane, but not others. The space between the outer and inner membranes is termed the perinuclear space and is connected with the lumen
