528 METAMORPHOSIS IN AMPHIBIANS 



Many hundreds of salamander larvae belonging to the species Amby- 

 stoma maculatum, Ambystoma opacum, A^nby stoma tigrinum, and 

 Eurycea rubra have been observed under the most varying conditions 

 during metamorphosis; but in not one instance were the gills reduced 

 to mere stubs without fringes before the shedding of the skin had 

 taken place. This time relation is observable even under conditions 

 of the greatest acceleration. Since it will become evident, and has 

 already been pointed out above that a similarly constant succession 

 in time between the two phenomena in question and other develop- 

 mental phenomena does not exist, we are forced to assume that a 

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

 the gills, which does not exist between these phenomena and other 

 developmental processes. Evidently both these phenomena must 

 have a common cause, and, indeed conditions which retard or pre- 

 vent the shedding of the skin also retard or prevent the reduction of 

 the gills, while conditions which accelerate or enforce the shedding of 

 the skin also accelerate or enforce the reduction of the gills. More- 

 over, it can be shown that both phenomena are enforced by the ap- 

 plication of iodine and inhibited by the lack of iodine; and conse- 

 quently they must be considered as constituting part of the amphibian 

 metamorphosis. Numerous experiments have been carried out to 

 test the action of iodine upon the shedding of the skin and the reduc- 

 tion of the gills; two are described below. 



A series of six laivse of Ambystoma opacum (Wi 1917) were kept in 

 ordinary tap water at approximately 25°C. and fed on earthworms. 

 5 weeks after hatching, the larvae had attained a length of 29.6 mm. 

 and were in an early larval stage. At this time they were placed in a 

 0.02 f)er cent solution of Bayer's iodothyrin in tap water. 8 or 9 

 days later, the skin was shed and the gills were reduced rapidly to 

 stubs without fringes, while the control larvae needed several weeks 

 more to undergo the same changes. This effect of iodine upon skin 

 shedding and the reduction of the gills was particularly conspicuous 

 as the development of other organs, e.g. the legs, did not make any 

 progress. Thus the phenomena of skin shedding and reduction of the 

 gills surely are caused by the action of iodine, and constitute what 

 we must call metamorphosis. 



