Chimaerie combinations of tissues from different species, genera or more widely separated groups have contributed important information to many developmental studies. Professor R. G. Harrison ('03) clearly established the value of such conibinations in his classical study of lateral line development in frogs by the use of tissues from two of the Ranidae, palusiris and syluaticn. His own subsequent applications of heteroplastic grafting procedures ('29, '35) as well as many others, too numerous and well known for documentation here, have proven the merit of such an approach to many problems. It seems most appropriate, therefore, to include an account of the application of this type of tissue combination to a study of limb development, another field which owes so much to Professor Harrison's pioneering work, in a volume dedicated to his memory.
The bulk of early evidence derived from experiments with limb discs reveals that thc presumptive limb mesoderm is the dominating component for early development of amphibian limbs. An ectodermal influence on digit shape was demonstrated (Rotmann, '33) but, in general, this layer was considered to be a relatively passive cover. Recent cxperiments (Saunders, '48 ; Saunders, Gasseling and Cairns, '59 ;