EDUARD UHLENHUTH 529 



In a previous paper^ it has been demonstrated that the feeding of 

 the thymus gland to larvae of Amby stoma opacum, Amby stoma macula- 

 turn, and Amby stoma tigrinum frequently retards and in rare cases 

 even inhibits metamorphosis, and that this inhibitory action of the 

 thymus diet is due to the fact that iodine is either completely absent 

 from the thymus or present in amounts so small as to make it impos- 

 sible for metamorphosis to take place at a normal time. In the 

 thymus-fed larvae not only the reduction of the gills but also the 

 shedding of the skin is either retarded or completely inhibited, and 

 yet other organs may develop normally and reach an almost adult 

 condition. This proves that while the development of the latter 

 organs is independent of the iodine action, both the skin shedding and 

 the reduction of the gills cannot take place in the absence of iodine. 



Relation between Development of Skin Coloration and Metamorphosis. 



Among the organs whose development is independent of metamor- 

 phosis, the structures which give rise to the coloration of the skin 

 are most conspicuous. Since the coloration of the skin is easily 

 observable without killing the animal, the development of this char- 

 acter can be used with great advantage to demonstrate its relation 

 tcr metamorphosis. The color development of the skin is most vari- 

 able as regards its time relation to metamorphosis. This relation is 

 subject to change under a number of different conditions; the devel- 

 opment of the skin coloration is not produced by the substances 

 causing metamorphosis, for it takes place without the action of 

 iodine. 



Though careful studies on the subject in question have not been 

 carried out thus far, the remarkable fact that gill-bearing larvae may 

 exhibit the coloration of metamorphosed animals has been observed 

 repeatedly. In 1891 von Bedriaga^ described in detail the colora- 

 tion of a number of larvae of tailed batrachians and frequently em- 

 phasized in this paper that neotenous larvae of the genus Triton may 

 possess colors similar to those of metamorphosed animals. Similarly 



8 Uhlenhuth, E., /. Gen. Physiol, 1918-19, i, 305, 473. 



9 von Bedriaga, J., Zool. Anz., 1891, xiv, 295, 301, 317, 333, 349, 373, 397. 



