AMPHIBIAN METAMORPHOSIS 6ij 



inducing metamorphosis in tadpoles. It has been supposed 

 that its failure to cause metamorphosis normally is due to its 

 possessing only a storage function, and not liberating its secre- 

 tion into the blood ; but this appears to be negatived by 

 experiments now being conducted by Hogben and myself. 

 Although grafting a th5'-roid from one individual to another 

 appears always to bring about the liberation of all or most 

 of the secretion, yet grafting of the thyroid of one, two, or 

 three animals into a single other specimen has been without 

 eifect on metamorphosis. It thus seems probable that although 

 the thyroid is almost certainly reduced in rate of development 

 and efficiency, yet it is largely a decrease in the sensitiveness 

 of the tissues to thyroid that has brought about the normal 

 failure to metamorphose in this animal. 



The pituitary, of course, constitutes a third independent 

 variable. If it has a rapid rate of development, it will tend 

 to bring about early metamorphosis, partly on account of its 

 own direct effect, mainly by exerting an effect upon the deve- 

 lopment of the thyroid ; conversely, slow pituitary develop- 

 ment will, ceteris paribus, bring about a delay in metamorphosis. 

 It is quite possible that other ductless glands may play an 

 opposing role— the islets of Langerhans, for instance, to whose 

 action on carbohydrate metabolism the thyroid appears to 

 be antagonistic. For the present we must be content to lump 

 such possible specific effects together under the head of " the 

 rest of the body." But it can be regarded as established 

 that the chief positive effects on metamorphosis are exerted 

 by the thyroid. 



From what we have said it appears clear (i) that the rate 

 of development of the thyroid, and its maximum size, can 

 vary genetically. This is shown positively in the three geo- 

 graphical races of one species investigated by Adler ; and is 

 the only reasonable hypothesis of the variation of time and 

 size of metamorphosis as seen, for instance, between a bull- 

 frog and an ordinary frog. (2) That the effectiveness of the 

 thyroid depends not on its absolute, but upon its relative size 

 or developmental rate ; and that the sensitiveness, etc., of the 

 tissues to which it is relative can also vary genetically (Champy). 

 (3) That both thyroid and condition of tissues can be affected 

 by environmental influences, e.g. temperature, or, in the case 

 of thyroid development, by the quantity of certain substances 

 (such as iodine) present in the food. (4) That when thyroid 

 concentration (or it would probably be more accurate to say 

 its result — basal metabolic rate) reaches a certain relative 

 level, metamorphosis is initiated by the breakdown of those 

 tissues which are adapted only to a lower rate of metabolism. 

 The final result of the quantitative interaction of the thyroid 



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