136 S. E. DETWILER 



The general results of these experiments indicate that when the 

 major portion of the gill tissue is removed and replaced by an 

 entire limb rudiment, the remaining unremoved portion of the 

 gill tissue, being in its normal organic environment, possesses a 

 relatively greater potency to produce normal gills than does the 

 entire limb system to produce a normal appendage — the latter 

 being in the heterotopic position. When a limb is transplanted 

 into a more passive region of the embryo such as that caudal to 

 its normal position, practically normal differentiation results. 

 For example, limbs reimplanted the distance of three body seg- 

 ments caudal to the normal location yielded 75 per cent normali- 

 ties (Detwiler, '20). When transplanted the same distance 

 anterior to the normal site, into the active self-differentiating 

 gill region, less than 15 per cent of the cases developed normally 

 (table 1). 



Case AA2S25. In this experiment (fig. 14) the initial develop- 

 ment of the limb was normal. Only one rudimentary gill de- 

 veloped, but the ectoderm in the vicinity of the base of the limb 

 wandered out over its flexor surface forming a permanent ridge 

 (figs. 14 and 15). The caudal border of the proximal portion 

 of the limb was also fused with the ectoderm of the gill, which 

 greatly restricted its motility. Function of the forearm and hand 

 were practically normal. 



Serial transverse sections showed that the glenoid cavity was 

 approximately two and one-half segments anterior to the nor- 

 mal position. The shoulder-girdle was abnormal. Its coracoid 

 portion was continuous with the fragmentary coracoid which 

 developed in the orthotopic position from unremoved portions 

 of its rudiment. The suprascapular portion was short and quite 

 thick. 



The shoulder muscles were poorly developed and were some- 

 what defective in nerve supply (table 3) , which was derived from 

 the ventral rami of the second, third, and fourth spinal ner\'es 

 (fig. 16). Although there was an abundant nerve supply from 

 the cord, a survey of the sections shows clearly that the incom- 

 pleteness of function in this case is due to the abnormal develop- 

 ment of the shoulder-girdle, deficiency of its corresponding mus- 

 culature, and the fusion of its proximal portion with the gill. 



