170 IODINE AND THE THYROID. IV 



ditions are different in anuran tadpoles. This writer says in this 

 regard : 



"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." 



This statement should be of interest to students of anuran meta- 

 morphosis who have experimented with iodine, as it is a denial of the 

 process, namely limb development, most obviously brought out by 

 iodine administration in these forms. There can be no question that 

 iodine has little or no effect upon limb development of urodeles, but 

 also there can be no doubt that this same substance stimulates to a 

 remarkable degree limb growth and formation in the Anura. Despite 

 the close phylogenetic relationship between Urodela and Anura there 

 exist some sharp distinctions between the larvae of the two groups in 

 regard to physiological responses to environmental conditions, and in 

 developmental history. Metamorphosis in the Apoda and Urodela is 

 restricted chiefly to the reduction of the gills, the closing of the clefts, 

 and the loss of the gill chamber and the finny margin of the tail; but 

 in frogs and toads, the change from the tadpole to the final form is a 

 fundamental reorganization of the organism, involving practically 

 every organ. The following quotation is from Boulenger:' 



In the newts the transformation is much more gradual {i.e., than the meta- 

 morphic change in Anurans) without the intercalation of a tadpole stage. The 

 final transformation in the Urodeles consists essentially in the loss of the gills 

 and the development of the eye-lids. I may mention that although the larvae 

 of Urodeles are often called tadpoles, the term as applied to them is quite mis- 

 leading, the larvae of frogs having an altogether different form and organization." 



Limb development in amphibians varies considerably in the differ- 

 ent types. Thus, in most Anura, the limbs develop slowly and require 

 months and years {Rana catesbiana) for complete development. In 

 other forms, such for instance as the urodele, Salamandra maculosa, 

 the young are born with legs. 



Uhlenhuth regards gill reduction and shedding of the skin in uro- 

 deles as constituting metamorphosis. It is evident that such a con- 



