AMPHIBIAN METAMORPHOSIS 607 



exert a metamorphic effect which is very much more marked 

 when the substances are iodised. It has been thought by some 

 workers (17) that these results with iodine and iodised amino- 

 acids indicate that the metamorphic effect of thyroid is entirely 

 related to its contained iodine, in contradistinction to the 

 well-known effect which it exerts in mammals upon the basal 

 metabohc rate. This, however, as later work shows, is a quite 

 gratuitous assumption ; the results in question are most 

 simply explained if we assume : (i) that iodine is normally 

 a hmiting factor in the growth of the larval amphibian thyroid, 

 (2) that both iodine and various amino-acids are " Bausteine " 

 of the true thyroid hormone, (3) that some or all of the normal 

 tissue-cells are also capable of elaborating thyroxin (or a very 

 similar active substance) but that the thyroid cells perform 

 this function in a much more specialised and efficient way. 

 In addition, as regards these results, it is a fact which is often 

 not taken into account by those accustomed to adult mam- 

 malian physiology, that the conditions during development 

 are very different from those obtaining during adult life, 

 particularly as regards regulation. Although external condi- 

 tions, such as temperature, do exert some effect upon the size 

 and activity of the tadpole's thyroid, yet, when iodine, etc., is 

 given there is no shutting down of the manufacture of thyroid 

 substance as occurs in adults, but the development of the 

 thyroid, other conditions remaining the same, is independent 

 of the rest of the body, dependent only on the amount of 

 " Bausteine " it receives. Thus any proportion between (a) 

 amount of thyroid hormone and (b) body-size and develop- 

 ment ma}^ exist in larval life, up to that concentration of 

 hormone needed to determine metamorphosis ; the actual 

 proportion found is dependent upon the treatment of the 

 animal, the food it receives, etc. 



The thyroid, however, is not the only gland concerned in 

 metamorphosis. The pituitary is also of importance, although 

 it is undoubtedly secondary to the thyroid (22, 23, 29). Tad- 

 poles from which the anterior lobe of the pituitary has been 

 removed also fail to metamorphose, while those into which 

 it has been engrafted show an acceleration of metamorphosis. 

 In anuran tadpoles this metamorphic effect of the pituitary 

 is exerted, at least in greater part, via the thj^oid. The 

 thyroid of pituitariless specimens is markedly underdeveloped, 

 but becomes normal if pituitary is engrafted. That the 

 pituitary may, however, exert a true metamorphic effect 

 independently of the thyroid is shown by the recent researches 

 of Hogben, who has succeeded in metamorphosing not only 

 normal but also thyroidectomised Axolotls by means of pre- 

 pituitary injections. However, it appears that a much heavier 



