Origin of endoplasmic reticulum. A) Cross-section schematic showing that the endoplasmic reticulum is an outgrowth of the “outer” nuclear membrane and makes close appositions with the plasma membrane. Note that this outgrowth does not alter how the nuclear pores connect the cyto- and nucleoplasms. B1) Schematic showing how in the late stage of proto-nucleus cell internalization, an exterior-medium filled “tail” continuous with the lumen surrounding the proto-nucleus cell could form. Dashed lines show how the tail would eventually be separated from the external medium by fusion of proto-cytoplasm cell membrane. Filled arrows represent non-respiratory (e.g., Na/K ATPase) pump activity. These pumps pump Na and Ca out of the cytoplasm into the external medium or lumen, and K out of the external medium or lumen into the cytoplasm. As the proto-cytoplasm cell surrounds the proto-nucleus cell, these pumps would thus automatically (i.e., without any change in their orientation in the membrane) work so as to maintain an external-like (high Na and Ca, low K) ionic environment in the lumen separating the cells. “L” shaped lines represent cell anchoring proteins; lines are grey because the nanotubes alone possibly could provide the physical support for internalization. Nanotubes are represented by the grey connections between the cells. Because this drawing is a slice through the cells, it appears that the nanotubes separate the lumen into compartments. However, because the nanotubes are cylinders, the lumen is actually continuous (see B2). B2) Three dimensional drawing of a thick slice (a slab) through the cells at stage B1 showing how the lumen and tail form a single, contiguous compartment. B1 and B2 are slices through the cells; in full three dimensional drawings, the proto-cytoplasm cell would surround the proto-nucleus cell on all sides and the tail would be a pipe-like structure.