A comparative EM study of the marginal zone of the neocortex in E15 foetuses of normal and 'reeler' mice was carried out. In the mutant a direct contact between 'proneurons' and the superficial basal lamina bordering the 'glia limitans' layer was observed. Since this phenomenon occurs with a much higher frequency in the mutant than in the normal animal, it is thought to be important in the genesis of the 'reeler' anomaly. In the discussion, different hypotheses concerning the possible nature, mechanism and importance of the interaction described are suggested.