Sammary-Epithelial mesenchymal interaction in the fusion process was studied by the means of fusing different extremities of a 15 day 16 hr rat foetus. The extremities used were the hands, feet, and tails, which at this stage of embryologic development appear to have a similar type of epithelial covering but diier markedly in the degree of specialization of the underlag mesenchyme. Two hands from one embryo were placed in palm to palm contact and grown under organ culture conditions for 72 hr with ten pairs of hands making up the experimental group. Ten pairs of feet made up the second experimental group with two feet from the same embryo being placed in sole to sole contact and grown under organ culture conditions. The third experimental group consisted of ten pairs of tails from littermate embryo's and were placed side by side and cultured for 72 hr. In all three experimental groups, fusion with epithelial breakdown occurred though to different extents. It was noted that the degree of mesenchymal penetration across the interrupted epithelial barrier was related to the extent of specialiition of the mesenchymal tissue. The greater the degree of specialization of the mesenchyme the less was the intermingling with the mesenchyme of the opposite member. The most specialized extremity the hand, also showed the lowest incidence of the fusion process progressing as far as the epithelial breakdown stage.