308 E. UHLENHUTH. 



Recent investigations have shown that the thyroid hormone and 

 probably other morphogenic hormones, by increasing the rate of 

 certain fundamental reactions, have the ability of causing struc- 

 tural changes throughout the entire organism, bringing thus about 

 morphological expressions of a wide range affecting nearly the 

 whole body. It seems that these hormonic substances, as far as 

 their immediate effect is concerned, act chiefly by inducing a gen- 

 eral histolysis throughout the various organs of the organism. 

 There can be no doubt, however, that, besides these hormones 

 referred to above, other substances play an important role in the 

 development of an organism ; these substances seem to possess a 

 more localized action, effect the development of only certain organs 

 and are concerned chiefly with the building up of the structures of 

 these organs. Certainly, in the evolution of the organisms, the 

 acquirement of the ability of elaborating the latter kind of sub- 

 stances must have played a role equally important as that played 

 by substances such as the thyroid hormone. The limbs of the 

 amphibians are apparently organs whose development seems to 

 depend chiefly on the action of substances belonging to the latter 

 group of substances and not on the activity of the thyroid hormone. 

 On account of the increased importance which, in the light of 

 such considerations, seemed to attach itself to the rinding that the 

 development of the salamander limbs is independent of the thyroid 

 hormone, it appeared necessary to repeat my previous experiments 

 on the relation between limb development and metamorphosis. 

 The present article will be devoted to reporting these new experi- 

 ments. They consisted in causing precocious metamorphosis of 

 the larvae of Ambystoma maculatum by keeping them in iodothy- 

 rine, whereby special attention was paid to a possible acceleration 

 of limb development. The result was the same as in the experi- 

 ments on A. opacuni; the rate of the development of the limbs 

 remained unchanged, although metamorphosis took place at an 

 early date. Not only larvae, but also embryos, at early stages, 

 were exposed to the influence of iodothyrine, in order to avoid the 

 objection that failure of the iodothyrine to cause accelerated limb 

 development was due to the experiments having been started at a 

 stage at which limb development was too far advanced. Again 



