316 E. UHLENHUTH. 



permit of the freeing of the fore limbs is identical with the change 

 that causes the first shedding of the skin. 



In support of this view is the fact that the freeing of the fore 

 limbs is clearly the only step in the development of the limbs, 

 which in not one single instance has been observed to take place in 

 tadpoles which had been deprived of the thyroid secretion, while 

 the developmental processes preceding the freeing of the limb may 

 take place in thyroidectomized tadpoles. Allen (10) states that in 

 tadpoles of Bufo, in the complete absence of a thyroid, both fore 

 and hind limbs develop normally (and even grow larger than the 

 limbs of normal larvae) i.e., behave exactly like the limbs of sala- 

 mander larvse yet the fore limbs fail to break through the walls 

 of the gill chamber. Apparently in this anuran species the de- 

 velopment of the limbs, except for the freeing of the fore limbs, 

 is independent of the thyroid hormone as it is in the urodelan 

 larvse. 



Should these views prove to be correct, it would seem probable 

 that the mode of limb development in amphibians is the morpho- 

 logic expression of the existence of two kinds of morphogenic sub- 

 stances ; one group of these substances serves to procure the actual 

 building stones of the morphological structures of the organ, while 

 the other group of substances merely brings about a general his- 

 tolysis of old structures, removing thus obstacles to the action of 

 the substances belonging to the first group of substances. 



As has been mentioned above, the ineffectiveness of inorganic 

 iodine in the limb development of salamanders has a reason differ- 

 ent from that of the ineffectiveness of iodothyrine. In a previous 

 article (5) I have shown that, in contradistinction to iodothyrine, 

 the administration of inorganic iodine does not produce precocious 

 metamorphosis in salamander larvse. The inorganic iodine has no 

 effect on the salamander metamorphosis, because the thyroid hor- 

 mone in salamanders is not released during the greater part of the 

 larval period, and a greater supply of inorganic iodine, even if it 

 should result in the elaboration of an excess of thyroid hormone, 

 as it actually does in tadpoles (n), can not make itself felt in the 

 salamander larvae in consequence of the retention of the hormone. 

 As has been shown in this article, inorganic iodine, like iodothyrine, 

 has no effect on limb development of salamanders. But it must 



