632 



Metamorphosis 



from the thyroid. This can be fractionated 

 to yield two active iodized derivatives of 

 tyrosine, namely di-iodo-tyrosine and thy- 

 roxine. The latter, being far more active than 

 the former in respect to thyroid-like proper- 



but fail to show any metamorphic changes 

 even if kept for many times the normal 

 larval period and until they exceed by far 

 the maximum size for control larvae (Allen, 

 '18; Hoskins and Hoskins, '19a). These re- 



Fig. 217. The pattern of metamorphosis in the frog, Rana pipiens. The graph shows the growth of 

 the hind legs (solid line) compared to that of the body (broken line). The scale on the ordinate is for 

 hindleg length; the abscissa scale is given in days, counting the day of emergence of the forelegs as 

 day 30. Metamorphic changes shown in sketches of tadpoles occur at times indicated. Metamorphosis 

 begins with differential acceleration of hind leg growth. When the legs are about half grown the tissue 

 in which the anal canal lies (anal canal piece, A.C.P.) begins to be absorbed and completely disappears 

 in two or three days. About this time an area of disintegration in the skin above the gills begins to 

 develop, becoming clearly defined in a few days (skin window for the forelegs, S.W.F.L.). When the 

 forelegs emerge the stage of early metamorphosis (prometamorphosis) goes over into metamorphic climax 

 when rapid morphological changes take place. The animal ceases to feed, the horny teeth and beaks (B) 

 and the associated fleshy mouth parts are lost and the mouth widens in a few days to that characteristic 

 of the frog. Tail resorption and eye growth proceed rapidly at this time. The last morphological altera- 

 tion is that of the formation of the tympanum (T). (After Etkin, '32; Taylor and Kollros, '46.) 



ties on all test animals, is often discussed as 

 though it were "the hormone" of the gland 

 (Means, '48). 



In the absence of the thyroid gland, meta- 

 morphosis fails to occiir. Extirpation of this 

 gland in the frog embryo was first accom- 

 plished by Allen ('16) and by Hoskins and 

 Hoskins ('17), who devised a simple tech- 

 nique whereby the primordium of the thy- 

 roid is removed at the tail-bud stage (Fig. 

 218). The operated animals grow normally 



suits have been confirmed and extended to 

 other species of Anura by many workers 

 (Allen, '38). So far as is known to the writer 

 no reports on the effects on metamorphosis 

 of removal of the thyroid primordium in 

 urodeles are available; in larvae, however, 

 total thyroidectomy suppresses metamorpho- 

 sis (Schwartzbach and Uhlenhuth, '28). 



The failure of metamorphosis after re- 

 moval of the primordium of the thyroid 

 gland in the anuran embryo is completely 



