3IO E. UHLENHUTH. 



were much like the hind limbs of an adult frog. Aloreover, in 



2 larvae of the iodine series, which died at an age of 50 and 56 

 days respectively, the hind limbs possessed already at that date 

 5 fully differentiated toes. 



In another experiment 5 series of the tadpoles of Rana sylvatica 

 were kept in different concentrations of iodine (varying from i to 

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

 The controls were kept under observation for 83 days ; at the end 

 of this period the hind limbs had remained undifferentiated, whitish 

 buds in 5 of the 8 larvae, while in the other 3 larvae differentiation 

 had taken place, the best differentiation being represented by 4 toes 

 on the foot of the hind limbs. Many of the larvae kept in inorganic 

 iodine died ; none of these was further advanced than the surviv- 

 ing larvae. Among the surviving larvae none had metamorphosed 

 at the termination of the experiment, nor had the fore limbs broken 

 through in a single instance ; yet the limbs were considerably fur- 

 ther differentiated than in the controls. In one larva of the iodine 

 series, at an age of 73 days, the foot of the hind limbs was differ- 

 entiated into 5 toes; in another larva, at an age of only 59 days, 

 the hind limbs possessed 5 toes, and the fore limbs, which could 

 be seen vigorously moving in the gill chamber, had developed 



3 toes. 



There can be no doubt that iodothyrine when administered to 

 tadpoles greatly accelerates development of the limbs. Inorganic 

 iodine, although it seemed less efficient in these experiments than 

 iodothyrine, likewise increases the rate of the development of the 

 limbs. 



EXPERIMENTS ON SALAMANDER LARVJE. 



As pointed out in the introduction, my previous experiments on 

 the larvae of Ambystoma opacuni, in which the administration of 

 iodothyrine did not accelerate development of the limbs, were open 

 to the criticism that the administration of iodothyrine was started 

 at a stage at which limb development was fairly advanced (the toes 

 having begun to differentiate), and that for this reason the iodo- 

 thyrine may have been incapable of accelerating limb development. 

 Therefore it seemed necessary to start the experiment at a very 

 early stage. Two experiments were carried out, both on the em- 

 bryos of Ambystoma maculatum. 



