EFFECT OF IODINE ON LARV.E OF SALAMANDERS. 315 



In tadpoles substances which cause precocious metamorphosis 

 also accelerate the development of the limbs. I have suggested, 

 in a previous article (8), that in spite of this external difference 

 existing between the larvae of Anura and Urodcla the primary 

 effect of the thyroid hormone may be the same in both groups of 

 animals, and the difference may not be a fundamentally different 

 reaction upon the thyroid hormone, but merely a different mode of 

 limb development. 



There is no doubt that, except for the development of the limbs 

 in tadpoles, the immediate effect of the thyroid hormone is, in both 

 groups, predominantly a breaking down of tissues throughout the 

 whole organism, not a building up of new organs. It is possible 

 that in tadpoles the same substances endowed with a merely local- 

 ized action cause limb development as in salamanders, but that in 

 tadpoles these substances can not commence to build up the struc- 

 tures of the limbs before some obstacle has been cleared away by 

 the action of the thyroid hormone. That the thyroid hormone 

 controls limb development in the tadpoles does not necessarily 

 mean that it has any part in the constructive processes of limb 

 development. If we consider the advanced stages of the develop- 

 ment of the fore limbs in tadpoles, we find conditions which make 

 it indeed very probable that the thyroid hormone, in this process, 

 plays merely the role of removing an obstacle external to the tissues 

 of the limb itself. In salamanders both hind and fore limbs de- 

 velop freely, while in tadpoles the fore limbs are inclosed in the 

 gill chamber. In order that they may break through the walls of 

 the gill chamber, certain changes of the skin and the tissues under- 

 lying it must take place ; these changes are not caused by the legs 

 themselves, but take place even in the absence of the limbs (9) at 

 the time at which metamorphosis occurs. I have pointed out re- 

 peatedly that in salamanders one of the most conspicuous effects 

 of the thyroid hormone is a certain change of the skin which finally 

 results in the shedding of the skin and may be identical with the 

 process which leads to the atrophy of the gills. A similar change 

 is brought about in the skin of the tadpoles; in the tadpoles, too, 

 the skin is shed for the first time when metamorphosis takes place. 

 It is possible that the change of the skin which is necessary to 



