TRAXSPLAXTATION OF LIMBS IN AMBLYSTOMA 151 



Typical gills failed to develop from the tissues lying dorsal to 

 the implanted limb rudiment, the ectoderm migrating out on the 

 dorsomesial surface of the developing appendage. It was in these 

 cases that the free appendage took on the posture of a developing 

 gill (fig. 6). In some cases the limb regained its normal posture 

 by a subsequent ventral rotation, but in the majority of the 

 experiments it underwent abortive development and perma- 

 nently assumed the pose of a typical external gill (figs. 26 and 

 29). 



That the outgrowth factors for the gills reside not only in the 

 ectoderm of the immediate gill region, but extend with diminishing 

 degree of intensity into the surrounding ectoderm, is evident from 

 these experiments. 



The relative gill-producing power of the ectoderm covering 

 specific contiguous outljdng regions (e.g., heart, pronephros, etc.) 

 could not be tested in the present work, but it is hoped that this 

 question may form the basis for a future investigation. 



Resume of experiments 



Viewing the experiments as a whole, we see that the limbs, 

 although transplanted from one to three segments anterior to 

 the normal position, received in most cases two or more nerves 

 from the original limb level of the cord (table 2). Although the 

 proximal portion of these nerves is seen to assume a normal course, 

 their distal portions have grown in an anterior direction, in spite 

 of mechanical opposition, and have effected functional con- 

 nection with the transplanted appendage (fig. 16, cf. fig. 4). 



The general function of these limbs was found to be less perfect 

 than in limbs .transplanted equivalent distances caudal to the 

 normal position (Detwiler, '20, table 1), which also received several 

 nerves from the original limb level of the cord (table 2). 



The more incomplete function of the limbs transplanted into 

 the gill region, as compared with that of limbs transplanted the 

 same distance caudal to the normal site, appears to be due to 

 greater structural deficiencies in the girdle and shoulder muscles 

 and to less complete peripheral innervation. The defective 



