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THE AM Kmc A .V .V. 1 TUIi. 1 LIST [Vol. LV 



proved to be little iiifluciiecd ])y tlie thyroid action. That 

 the development of the li]nl)s of salamanders is not de- 

 pendent on the thyroid gUmd has been pointed out above; 

 here I may add that Typhlomolge is a further illustration 

 of this fact, as in this salamander the legs develop in a 

 normal manner in si)ite of the complete absence of the 

 thyroid gland. The rclatioi, of limb .b'veh.pmciit and 

 thyroid action in tadi).)lcs is by ii<. means dclinitely 

 settled as yt. In ladpuh-s thr development of the limbs 

 seems to be hiuhly ,le},en.lent on the action of the thyroid 

 gland; but attention ha^ l)een ealled to this snr|)ri>ing 

 difference between two gi-oups of oi'ganisms so ('h)sely 

 related otherwise and the suggestion has l)een made in a 

 previous article (18), that this ditference as far as the 

 fore legs are concerned may be due merely to the fact 

 that in tadpoles the limbs grow beneath the skin and 

 consequently can not break through unh'ss the changes 

 are initiated which finally lead to tlie she<ldin-' of the skin 

 and that these changes and not thr thyroid actiim are the 

 primary factor in the develo])nient of tlie amiran fore 

 limbs. Whether or not this a>snm]iti(ui is correct can 

 not be decided at present, but cei-tainl\- (h'serx'es I'eiiewed 

 attention in view of recent discoveries which ( h'tnonst i-ate 

 that the development of the liml)s of tadpoh's. at h-asi in 

 certain species, is not as dependent on tin' th\ l oid >eero- 

 tion as some investigators were inclined to think. Allen 

 (34), who has made prolonged observations in thyroidec- 

 tomized tadpoles, has recently found that not only the 

 hind limbs, but even the fore limbs in the thyroidectom- 

 ized larvae of Bufo ultimately attain a size and differenti- 

 ation not only equal but superior to those attained in nor- 

 mal metamorphosing larvae. The only difference, how- 

 ever, is that in the absence of the thyroid gland the fore 

 limbs can not bi-eak tlirongli the skin. 



As to tlie skin pIutMriitntion, it is wll known that larva^ 



