EDUAED UHLENHUTH 527 



Relation between the Reduction of the Gills, the First Skin Shedding, and 



Metamorphosis. 



It has become customary to apply the term metamorphosis to the 

 processes which can be enforced by thyroid treatment. Though it 

 is claimed frequently that in tadpoles the action of iodine can be 

 defined accurately, it seems that certain phenomena have been in- 

 cluded as a result of iodine action which in reality are not due directly 

 to this action. It will be shown that the effect of iodine can be de- 

 termined most accurately in salamanders, at least as regards two 

 organs, the gills and the skin, while the development of several organs 

 can be shown to be independent from the iodine mechanism. 



In salamanders, two phenomena are most conspicuous during 

 metamorphosis ; the first shedding of the skin and the reduction of the 

 gills. 



In temperatures of from 20-30° C, the first shedding of the skin 

 never takes more than 24 hours and frequently only from 2 to 5 hours. 

 In lower temperatures it may take from 1 to several days. But in 

 both cases the beginning and the end can be determined with accuracy 

 if the animals are watched carefully. It is thus possible to observe 

 accurately the time relation which exists between the first shedding 

 of the skin and other developmental phenomena. While several of 

 the latter phenomena have been observed to take place either before 

 or after the first shedding of the skin, as will be discussed below, the 

 time relation between the shedding of the skin and the reduction of 

 the gills is constant and unchangeable. The condition of the gills 

 (external in salamanders) varies greatly and is influenced much by 

 the quantity of food available to the larvae, as Powers'^ has stated. 

 Well nourished larvae possess large gills with long fringes; in poorly 

 nourished animals the stems as well as the fringes become short. 

 But in whatever state the larvae may be kept, the gills are always 

 characterized by the possession of stems as well as fringes. In meta- 

 morphosis, however, the gills are reduced to short stubs without 

 fringes. The transition from the state with fringes into that with- 

 out fringes is sudden and definite, and requires only a few hours. 



^ Powers, J. H., Am. Nat., 1903, xxxvii, 385. 



