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The objects of this work were to reinvestigate and extend the results announced in a brief note published in 1925 (Murray and Huxley, 1925a). In this paper it was concluded that isolated fragments of the limb buds of the fourday chick are able to self-differentiate when Iiving as grafts on the chorio-allantoic membrane of older chicks; that the bud of the four-day chick is a mosaic; it was hinted that each of the morphological regions of the limb (femur, tibia-fibula, etc.) is represented by a Single piece of the mosaic; that no regeneration or regulation occurs in fragments of the bud, except that it was concluded that if a grafted fragment contains only part of a piece of the mosaic, that part could so regulate its future development as to form the complete morphological region of which it was originally a part, so that a fragment of the bud which contains part only of the region Which would normally form femur will, when growing as a graft, form a complete femur. It will be seen that the results of the present work uphold and confirm the tentative conclusions previously advanced, except in regard to the last point (regulation). A considerable amonnt of further information has also been obtained bearing on tile factors concerned in the development of the form of bones and joints.