PERMEABILITY OF MEMBRANE JUNCTIONS *

The general and familiar idea of the cell membrane is that of a barrier to diffusion encompassing the entire cell surface. This idea derives largely from work with a few cell types: nerve, striated muscle, blood, and gamete cells. These are, however, rather peculiar cells. They are generally not in direct contact with each other but are separated by wide fluid-filled spaces. I shall deal here with the question of what happens to the diffusion barrier when two cell membranes are brought close together. This states the question for the general case of cell association; most tissues, by far, are built of cells in close surface contact. I chose epithelial cells for a study of this question. These cells have several advantages. Their areas of membrane contact are large, and their size permits direct conductance measurements. The giant salivary gland cells of Drosophila and Chironornus were particularly suitable in this respect (FIGURE 1 ) .