118 S. R. DETWILER 



ANATOMICAL 



Since these transplantation experiments have been carried out 

 from the standpoint of the nerve connections and functional 

 behavior of the limbs, it becomes necessary, for proper discussion 

 of the results, to consider briefly the anatomical factors involved 

 in the regulation of the motiht}^ of the heterotopic appendage. 

 Four such factors have been discussed previously (Detwiler, '19, 

 '20) : — 1) the completeness of the shoulder-girdle in the hetero- 

 topic position, 2) the degree of differentiation of the shoulder and 

 limb muscles, 3) the completeness of peripheral nerve connections 

 with the above muscles, and, 4) the character of the connections 

 within the central nervous system. 



1. Shoulder-girdle 



Since the shoulder-girdle has the character of a mosaic 

 (Detwiler, '18), its degree of development in the transplanted 

 position is variable, depending on the size of the graft and the 

 region from which it is taken. When a typical limb rudiment is 

 transplanted (fig. 1), only the tissue normally developing into 

 the more central portion of the girdle is included. The localized 

 rudiments of the more outlying portions of the girdle (supra- 

 scapula and the ventral portion of the coracoid) lie beyDnd the 

 limits of the tissue typically included in the limb graft, and con- 

 sequently undergo development in situ following excision of 

 the transplant. The girdle which develops in the heterotopic 

 position is always reduced in size and is qualitatively incomplete. 

 There is considerable evidence in many cases, however, to show 

 that compensatory hyperplasia from the dorsal and ventral 

 portions of the reduced heterotopic girdle takes place as the 

 larva matures, so that the final size of the girdle may almost 

 equal that of the normal. Such cases will be referred to later. 



No conclusive evidence has been obtained from transplantation 

 experiments in Amblystoma to show that the girdle rudiment in 

 this form, in spite of its developmental intimacy with the limb, 

 constitutes a totipotent system such as the limb itself is. Braus 

 ('09, p. 271), however, maintains that in Bombinator the girdle, 

 like the limb, constitutes an equipotential restitution system. 



