EDUARD UHLENHUTH 541 



Two reasons may be assumed to account for the inhibition of meta- 

 morphosis in Amhy stoma tigrinum. First, it is possible that with pro- 

 gressing age certain changes occur in the organism which abolish the 

 susceptibility of the larvae to the action of iodine; but experiments 

 have shown that even very old larvae can be forced to metamorphose 

 by direct application of iodothyrin. 



Second, it is possible that with progressing age certain changes 

 occur in the thyroid gland which make this gland progressively less 

 susceptible co the action of the excretor substance. By this assump- 

 tion the problem can be solved. The localities where the perenni- 

 branchiate form of Ambystoma tigrinum occurs are all located high in 

 the mountains and consec{uently the temperature of the water in 

 these lakes is very low. As in other cases it is possible that the 

 temperature coefficient of the thyroid change is lower than that of 

 the excretor formation, and that for this reason lowering of the tem- 

 perature decreases the rate of the excretor formation more than it 

 decreases the rate of the thyroid change. Thus at the time when 

 the quantity of excretor required under normal conditions to induce 

 thyroid excretion would be evolved, the thyroid would be in a state 

 where this quantity would not any longer be sufficient to affect ex- 

 cretion. If the rate of the thyroid change which makes this organ 

 less and less sensitive to the action of the excretor, and the rate of 

 the excretor formation would be constant under these conditions, 

 the difference between the quantities of the two factors would increase 

 progressively, and metamorphosis would become more and more 

 impossible. 



The development of the yellow bands of Ambystoma tigrinum in low 

 temperature is indeed very similar to the metamorphosis of the same 

 species, both becoming inhibited permanently by low temperature; 

 the only difference is that inhibition of the development of the yel- 

 low bands can be effected at a temperature somewhat higher than 

 that at which metamorphosis can be inhibited. From this simi- 

 larity we may conclude that the development of yellow bands in 

 Ambystoma tigrinum is due to an interaction of factors similar to but 

 not identical with those involved in metamorphosis. Possibly one 

 of these factors is a structure which loses in susceptibility towards a 



