A survey of a large number of different cell types has indicated the presence of a network of membrane-bound cavities (the endoplasmic reticulum) in the cytoplasm of all cell types examined, with the exception of the mature erythrocyte.

In its simplest form, encountered in seminal epithelia and in leucocytes, the reticulum consists mainly of interconnected strings of vesicles and appears to be randomly disposed in three dimensions.

Local differentiations occur within the endoplasmic reticulum of all the cell types studied.

The membrane limiting the cavities of the endoplasmic reticulum appears to be continuous with the cell membrane and the nuclear membranes.