126 S. R. DETWILER 



of the ectoderm in the general region surrounding the trans- 

 planted rudiment. Developmental conflicts arose between the 

 gill-forming ectoderm and the limb mesoderm, producing not 

 only various distortions in both gills and the limb, but frequently 

 resulting in almost complete suppression of the latter. 



Further experiments showed that complete limb differentiation 

 in the gill region could not be expected with any certainty unless 

 complete suppression of the gills was first accomplished by ex- 

 cising much larger areas prior to making the transplantation. 

 Several cases were obtained, however, in which normal limb 

 differentiation did take place, even though one or two atrophic 

 gills did develop. 



Series B 



In the second series of experiments the ectoderm and mesoderm 

 from the entire gill swelling was removed (fig. 3). The trans- 

 planted limb rudiment in these cases occupied a position ap- 

 proximately the distance of three segments anterior to the normal 

 (fig. 3) . The excavated limb region in all cases was covered with 

 indifferent ectoderm taken from the caudal portion of another 

 embryo. The results of these transplantations upon the develop- 

 ment of the limbs and the gills are summarized in table 1 B, 

 Although the ectoderm and mesoderm from the entire gill swell- 

 ing was removed, the tabulation shows that 55 per cent of the 

 cases developed abortive and abnormal gills. That these struc- 

 tures develop from tissue lying beyond the confines of the immedi- 

 ate gill swelling is without doubt, and this observation confirms 

 that of Ekmann (op. cit.). Because of the inherent ability of the 

 surrounding ectoderm to produce gills, the percentage of ab- 

 normal limbs was high. Even when only one or more abortive 

 gills were formed, the ectoderm surrounding the base of the 

 developing limb frequently migrated out upon the appendage, 

 causing it to become secondarily fused to the side of the body wall 

 (fig. 14). Such limbs, although structurally complete and with 

 considerable nerve supply, were unable to enjoy freedom of 

 movement. 



