The endoplasmic reticulum was first observed through a microscope in the mid 1940s. The organelle consists of an extensive interconnected network of flattened sacs and branching tubules that stretch throughout the cytoplasm and share a single continuous membrane. The internal space, or lumen, of the ER is large and highly convoluted. It typically comprises 10 percent or more of total cell volume. The extensiveness of the ER network was visualized in the digital video sequences in this section with a mEmerald-ER targeting signal fusion tag. Mitochondria in the fox lung cells were also visualized through the use of mCherry fused to a mitochondrial targeting signal.