Ill] THE THYROID GLAND . 265 



experiment has been performed on ordinary tadpoles*; with their 

 thyroids removed they remain normal to all appearance, but the 

 weeks go by and metamorphosis does not take place. Gill-clefts 

 and tail persist, no limbs appear, brain and gut retain their larval 

 features; but months after, or apparently at any time, the belated 

 tadpoles respond to a diet of thyroid, and may be turned into frogs 

 by means of it. The Mexican axolotl is a grown-up tadpole which, 

 when the ponds dry up (as they seldom do), completes its growth 

 and turns into a gill-less, lung-breathing newt or salamander!; but 

 feed it on thyroid, even for a single meal, and its metamorphosis is 

 hastened and ensured {. 



Much has been done since these pioneering experiments, all going 

 to shew that the thyroid plays its active part in the tissue-changes 

 which accompany and constitute metamorphosis. It looks as though 

 more thyroid meant more respiratory activity, more oxygen- 

 consumption, more oxidative metabohsm, more tissue-change, hence/ 

 earlier bodily development §. Pituitary and thyroid are very different 

 things; the one enhances growth, the other retards it. Thyroid 

 stimulates metabolism and hastens development, but the tissues 

 waste. 



It is a curious fact, but it has often been observed, that starvation 

 or inanition has, in the long run, a similar effect of hastening 

 metamorphosis II . The meaning of this phenomenon is unknown. 



An extremely remarkable case is that of the "galls", brought 

 into existence on various plants in response to the prick of a small 

 insect's ovipositor. One tree, an oak for instance, may bear galls 



* Bennett Allen, Biol. Bull, xxxii, 1917; Journ. Exp. Zool. xxiv, 1918; xxx, 

 1920; etc. 



t Colorado axolotls are much more apt to metamorphose than the Mexican 

 variety. 



J Babak, Ueber die Beziehung der Metamorphose . . . zur inneren Secretion, 

 Centralbl f. Physiol, x, 1913. Cf. Abderhalden, Studien iiber die von einzelrien 

 Organen hervorgebrachten Substanzen mit spezifischer Wirkung, Pfliiger's Archiv, 

 CLXii, 1915. 



§ Certain experiments by M. Morse {Journ. Biol. Chem. xix, 1915) seeihed to 

 shew that the effect of thyroid on metamorphosis depended on iodine; l;^t the 

 case is by no means clear (cf. 0. Shinryo, Sci. Rep. Tohoku Univ. iii, 1928, and 

 others). The axolotl is said to shew little response to experimental iodine, and 

 its ally Necturus none at all (cf. B. M. Allen, in Biol. Reviews, xiii, 1939). 



!| Cf. Krizensky, Die beschleunigende Ein wirkung des Hungerns auf die Meta- 

 morphose, Biol. Centralbl. xi.iv, 1914. Cf. antea, p. 170. 



