This symposium honors and memorializes Dr. Homer W. Smith. To it he contributes a masterly historical introduction and names Carl Näegli as the discoverer of the plasma membrane, that peculiar layer marking the boundary between protoplasm and its environment. This layer traditionally is endowed with remarkable properties and significance. These properties are here explored in depth: physical behavior, ultrastructure, interaction with ions, and excitability. The results are clearly and attractively reproduced.