No. 638] INTERNAL SECRETIONS OF AMI'IIiniAXS 199 



siderable destruction of tlio cartilag-inoiis visceral skele- 

 ton, the atrophy of the larval moutii in anurans, the 

 reduction of the intestinal coils in anurans, the complete 

 atrophy of the tail in anurans and the atrophy of the fin 

 in urodelans. Not before this extensive breakdown of 

 the larval tissues has taken place and out of the renniants 

 of the destroyed organs the new origans of llic adult 

 develop. Particularly instnictive in this iv-aid is 

 the development of the epithelial bodies in ilic of 

 salamanders; these develop from tln^ cj)!! hcliiiin of the 

 destroyed gills and in the midst of the ni;i»( s ..1" (Irti iliis 

 which result from the destruction esix-ciallv of tlif i:ill 

 vessels. 



The fact that metamorphosis can be bi-oiiuht al)out hy 

 feeding mammalian thyroid substance to the aiiipliihian 

 larvaB, docs not of course prove that the aiuphihiaii iiicta- 

 morpliosis, under normal circumstances, is the result of 

 the function of the amphibian thyroid gland itself. This, 

 however, is the case, as demonstrated especially by the 

 work of Allen (9) and of E. 1^ and :\r. M. TToskins '( 10). 

 If in an early embryonic stage of the anuran oru'anisni 

 the thyi-oid i> ..\tii-pate(l, nietaniorphoH. (-an not take 

 place at all and the tadpoles remain permanently (as far 

 as the ol.M i-valioii. uo) in the Mau- <.f an aMnatIc ani- 

 ])hil)ian lai-Na. Crowtli likewise nltiina'.^lv interfered 

 with, althonuii the t h\ roi.lretoniizr.l ta.lpolr. may urnxv 



normal ta.li'.ole.. On the other han^l. if the thyroid of 

 metann.rphie ta.lpo]e> i. -rafted t._. tadpoh-. ^^h\vh are 



the stauc of llu' la.rwn from which the thyroid graft was 

 taken, is caiixMl (11). The metamorphosis of the ani- 

 ])liil)ian e\-e is likewise impossible if it is removed from 

 the intbnMU't' of the tliyroid ln>rmone which controls the 

 developmrnt of the eM.. If <'y,.. of ol.l >alamaiuler larv;r 

 are -rafted to yuni- larv;e. i!„. metamorphosis of the 

 graft ]iiav be retarded l.y a> many as 7 months ami will 



