EFFECT OF IODINE ON LARV-E OF SALAMANDERS. 309 



these experiments were completely negative as to an acceleration 

 of limb development. In order to be certain that the method 

 employed in my experiments on salamanders was correct, several 

 tests were carried out on tadpoles ; these were positive. 



In addition to these experiments, several experiments were 

 made to test the influence of inorganic iodine on the development 

 of the amphibian limbs. Like Swingle (4), I found a distinct 

 acceleration of the limb development in tadpoles ; in the salamander 

 larvae, however, iodine had no effect on the development of the 

 limbs. The bearing of this result, which is different from that of 

 the experiments with iodothyrine will be referred to in the dis- 

 cussion. 



EXPERIMENTS ON TADPOLES. 



The experiments on tadpoles, which were intended to serve as 

 a check to the experiments on salamanders, were not advanced 

 beyond a merely preliminary stage, since they gave positive results 

 from the very beginning. As they are confirmative of the observa- 

 tions reported by other investigators, they will be reported only 

 briefly. 



In one experiment on the larvae of Rana sylvatica the controls 

 were kept under observation till the 66th day after hatching. At 

 this time the hind limbs of the control larva furthest advanced 

 possessed 3 distinctly differentiated toes, while the 2 other toes 

 were still rudimentary. In the iodothyrine series (kept from the 

 1 8th to the 26th day in water which contained 0.005 g m - Bayer's 

 iodothyrine per 1,000 c.c. of water) the fore limbs broke through 

 the walls of the gill chamber on the 33d day after hatching in 

 every one of the 3 larvae surviving, at this date, from the 6 larvae 

 composing the series at the beginning of the experiment. The in- 

 organic iodine, in the concentration used in this experiment (2 to 

 3 drops of a 1/20 M solution of iodine per 1,000 c.c. of water), 

 as well as in other experiments, proved to be considerably less 

 effective than the iodothyrine, as 66 days after hatching the fore 

 limbs had not perforated the gill chamber in a single instance. 

 Yet the limbs, the hind limbs as well as the fore limbs, were dis- 

 tinctly further differentiated than in the controls of the same age ; 

 the hind limbs possessed 5 fully differentiated toes and in shape 



