Na2 35SO4 was either injected to pregnant mice or added to a "Wolff and Haffen" culture medium. In various experiments, palatine shelves and half-lips remained one hour on this labeled culture medium. They were thereafter associated to another labeled fragment, either to their homologous shelf in a homotypical culture, or to a half-lip in heterotypical culture. In the same way, labeled half-lips were coupled to unlabeled palatine shelves. In all these cases, the mesenchyme was labeled by the tracer while the epithelial cells remained empty. In the medio-palatine epithelium, some dislocations could be noticed between the cells. These gaps were filled by the labeled mesenchyme with which they were in continuity. At this level the tracer appeared on the external surface. In the homotypical associations, it invaded, through these channels, the labeled antagonistic explant; in the heterotypical associations, lips plus palatine shelves, the labial epithelium prevented all the penetrations, except when alterations occurred, due to a long lasting culture. The role of the mesenchyme in the process of fusion and in the possible building of a surface coating is discussed.