534 METAMORPHOSIS IN AMPHIBIANS 



It is remarkable, however, that in two so closely related groups of 

 animals as the caudate and tailless batrachians, the same process 

 should be caused by so different a mechanism. It is difficult to refute 

 the suspicion that in the Salientia the development of the limbs may 

 be initiated only indirectly by the iodine action, perhaps on account 

 of some incidental anatomical structure which first must be broken 

 down by the autolytic action of the iodine. And, in fact, there exists 

 such a structure in the Salientia, which may be in the way of the 

 development of the fore limbs at least. It is well known that in the 

 frogs and toads the fore legs begin to develop long before they become 

 visible externally; they develop enclosed in the gill chambers and 

 covered by the skin of the body. Would it not be possible that the 

 autolytic changes of the skin which finally lead to the first shedding 

 of the skin and which are actually effected by the iodine action, must 

 first be initiated by the iodine in order that the legs can be freed and 

 full development can take place in the tadpoles? In this connection 

 the experiments which Lenhart^^ performed on tadpoles in order to 

 test the action of thyroid glands of varying iodine content may be 

 mentioned. He noticed that with increasing iodine content the ab- 

 sorption of the tail takes place more and more c^uickly while dif- 

 ferentiation, on the contrary, becomes less, taking the development 

 of the limbs as expression of the differentiation. This result can be 

 understood only if we assume that the autolytic processes alone lead- 

 ing to absorption of the tail, shedding of the skin, and metamorphosis, 

 are caused directly by the action of the iodine. If the time between 

 the beginning of the autolytic processes and their final result, the 

 shedding of the skin, is long enough, tJie legs have time to develop 

 after they have been freed, before metamorphosis takes place. But 

 if by means of strong iodine concentration this interval becomes 

 much shortened, the legs have no time to differentiate before meta- 

 morphosis occurs, the iodine, the concentration of which- was increased 

 artificially, acting more strongly than the substances causing leg 

 development, which were not increased artificially. 



That leg development is not a part of the amphibian metamor- 

 phosis is also demonstrated by the fact that in certain amphibians, 



1^ Lcnhart, C. H., /. Exp. Med., 1915, xxii, 739. 



