Fluorescein-sodium, a fluorescent tracer whose molecular weight is 376, diffuses rather freely from the interior of one cell to another in a gland epithelium (Drosophila) but does not diffuse along the intercellular space to the exterior. The permeability of the junctional surfaces of the cell membranes appears to be high, in contrast to the nonjunctional surfaces and intercellular spaces which represent strong diffusion barriers.
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Kanno et al. (1964) studied this question.
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