314 E - UHLENHUTH. 



the controls. Since, however, metamorphosis took place at an age 

 at which normally the limbs are fully developed (as shown by the 

 controls), the precociously metamorphosed salamanders possessed 

 in this experiment fully developed limbs. 



Concerning the influence of inorganic iodine in these experi- 

 ments, it was shown in a previous article (5) that administration 

 of iodine does not result in precocious metamorphosis of salaman- 

 der larvae. In this article it should be added that it did have no 

 effect also on the development of the limbs. 



DISCUSSION. 



The experiments reported in this article confirm fully the obser- 

 vations made in my previous experiments. In the larvae of sala- 

 manders the development of the limbs can not be accelerated by 

 the administration of iodothyrine. Therefore, if iodothyrine is 

 administered in doses which cause metamorphosis before the time 

 at which, under normal conditions, the limbs are fully developed, 

 metamorphosis takes place before the completion of limb develop- 

 ment. 



These facts demonstrate that in salamanders limb development 

 is independent of -the substance (thyroid hormone) which causes 

 metamorphosis. This conclusion has recently been supported by 

 several other facts. Typhlomolge rathbuni, the Texan cave sala- 

 mander, does not possess a thyroid gland (6) and consequently 

 does not metamorphose ; yet its limbs develop in a normal manner. 

 Hoskins and Hoskins (7) have shown that in the larvae of Amby- 

 stoma the limbs develop normally, if the larvae are deprived of 

 their thyroid glands in early embryonic stages. This season I have 

 repeated these experiments. Larvae of Ambystoma maculatum 

 were thyroidectomized at an early embryonic stage; these larvae 

 which are believed to have been successfully operated on (histologi- 

 cal examination has not been made as yet) showed the same rate 

 of limb development as the controls. Several larvae of Ambystoma 

 tigrinum were thyroidectomized at a stage at which 3 toes of the 

 hind limbs were developed ; the two other toes developed at a 

 normal rate after thyroidectomy. These facts prove that in sala- 

 manders the substances causing limb development are not identical 

 with those causing metamorphosis and consequently are not identi- 

 cal with the thyroid hormone. 



