Huxley : Glands and Development 



355 



METAMORPHOSIS COMPLETE 



Figure io. The dorsal fin and external gills have disappeared, the shape of the head 

 has changed and the eyes protrude. There is also a marked loss of flesh. The thyroid 

 is not specifically a metamorphosisHproducing agent, and its effects are probably confined 

 to speeding up metabolism. The time of metamorphosis in different species is determined 

 by the ability of the larval tissues to hold together under the intensified metabolic pro- 

 cesses set up by greater concentration of thyroid. 



can exist independently of age or size 

 when no thyroid is present and that 

 therefore age or size per se has no in- 

 fluence upon metamorphosis. 



The normal tadpole, however, at an 

 age when only the rudiments of limbs 

 are present, possesses a well-developed 

 thyroid. Swingle,"^ working with 

 neotenous bull-frog tadpoles, i. e., in- 

 dividuals in which the metamorphosis 

 is abnormally delayed for one or more 

 years, found that their thyroids were 

 perfectly capable of inducing meta- 

 morphosis when grafted into tadpoles 

 of small species. Further he tried 

 the same experiment with the thy- 

 roid of the normally neotenous Axolotl 

 and found that it, too, could cause 

 precocious metamorphosis in frog 

 larvae. 



Perhaps most remarkable of all is 

 the fact that in permanently neotenous 

 forms, such as Necturus, Hogben and 

 Swingle have independently shown 

 that no amount of thvroid feeding," 



nor thyroid feeding and grafting, to- 

 gether with pituitary treatment,'' will 

 bring about metamorphosis. The ut- 

 most that has been observed with any 

 perennibranchiate form, as a result of 

 thyroid treatment, is a slight reduc- 

 tion in height of the fin. On the 

 other hand. Swingle has shown that 

 the thyroid of Necturus will produce 

 metamorphosis when giafted on An- 

 uran larvae. It has further been 

 found impossible to produce meta- 

 morphosis in Lampreys by thyroid 

 treatment of the Ammiocoete larvae.'" 



The first thing that is clear is that^ 

 the thyroid is not a specific "meta- 

 morphosis-producing" agent. In some 

 metamorphosing animals it does, in 

 others it does not produce metamor- 

 phosis. Furthermore, recent work of 

 Hogben shows that injection of an- 

 terior lobe of the pituitary causes me- 

 tamorphosis in thyroidless as v/ell as 

 in normal Axolotls, so that the effect 

 of the pituitary upon metamorphosis 



