1. A microscopical analysis was made concerning the differentiation of ectoderm cut from the tip of the tail-bud of an amphibian embryo (Amblystoma mexicanum, stages 35-37, Harrison) after its implantation into the primordium of the limb-bud of a host embryo 3-5 weeks after operation.

2. The ectoderm which lay deep in the tissues of the limb differentiated either into solid epithelial cords or into cysts.

3. The ectoderm which was attached outside the limb differentiated into notched ectodermal elevations which included a mesenchymal core.

4. A microscopical analysis was made concerning the development of deformities of limbs as the result of the operation.

5. The scapula may be divided into isolated pieces, bundles of muscle fibres separating the pieces from each other.

6. A supernumerary piece of cartilage can develop close to the cartilage of the scapula.

7. The suprascapula may be absent and its place taken by a mass of muscle fibres.

8. A phocomelias may be produced when the whole length of the humerus and the elbow-joint lies inside the body wall. In this case the implanted ectoderm covers the area where the limb would normally develop.

9. The humerus may be reduplicated.

10. The humerus may be too short.

11. The proximal half of the humerus may possess a diameter different from that of the distal half.

12. One skeletal element only of the forearm (radius or ulna) may be present when the place which would normally be occupied by one of these elements was taken by implanted ectoderm.

13. The elements of the carpus and of the hand may appear irregularly scattered throughout the tissues of the distal part of the limb. In these cases the implanted ectoderm was attached to the surface of the distal end of the limb.

14. The fingers can show: (a) abnormal positions, (b) abnormal numbers, (c) syndactylias, (d) one finger too long, others too short.

The work was aided by a grant from the Medical Faculty Research Fund, University of Birmingham, for which the author wishes to tender his most grateful thanks.