206 THE AMERICAN NATURALIST [Vol. LV 



larvie, their metamorphosis is inhibited until the host 

 metamorphoses. If the eye graft, however, is taken from 

 larvfB which are near metamorphosis, such an inhibition 

 is no longer possible. Apparently shortly before meta- 

 morphosis actually occurs, the thyroid begins to excrete, 

 and after the circulating hormone has reached the eye 

 metamorphosis of the eye takes place, even if the, organ 

 is transferred to an animal in which the thyroid hormone 

 has not yet been secreted. 



It is quite possible, that the late beginning of the thy- 

 roid function in salamander larvae is one of the causes 

 why the administration of an excess of iodine is ineffec- 

 tive in the metamorphosis of these amphibians. Probably 

 the thyroid merely stores up the excess of iodine, but 

 does not release the hormone till shortly before the first 

 moult. 



Allen (19) has recently examined the condition of the 

 thyroid of Colorado axolotls and has found that they 

 possess a thyroid corresponding in size, structure and 

 colloid content to the thyi-oid of a(hilt spoeimens ol" A. 

 finrnniw. The thyroid ..'f ihc h\v\-v of otlier sa];mi;ni(h'r 

 specie's likewise seems te he i.iatinv inueh before meta- 

 morphosis aetualiy takes place. .VMeii coiiehnh'd from 

 his observations that tiie thyroid of salamantk^rs begins 

 to function at an early stage of the hirviL\ The inde- 

 pendence of the larval development of the salamander 

 larva' as demonstrated by the facts mentioned above 

 shows, however, that the presence of a matui-e thyroid 

 before metamorphosis must l)e interi)i<'te(l in a different 

 way. The most conspicuous character in the salamander 

 metamorphosis is tlie fact that, ahliougli it certainly is 

 dependent on the thyioid horinoiic. It does not necessarily 

 take place in larva^ whose iii> loid is mature. This can 

 only mean that two factors are reijuired in order to bring 

 about the metamorphosis of salaniandei- larva-, namely a 

 mature thyroid gland and a factor \\\\\c\\ icleases the 

 thyroid hormone from the follicles of the uland. 



This conception, which is now supported b\- several 



