532 METAMORPHOSIS IN AMPHIBIANS 



1 and 2), and was fully developed from 5 to 6 weeks after metamor- 

 phosis (Fig. 3). In Series B and F the gray network of the adult 

 had started to develop before metamorphosis. In five larvaj whose 

 metamorphosis was either checked completely or retarded to a con- 

 siderable extent, the gray network reached a condition very similar 

 to the adult condition as shown in Fig. 4. 



These experiments have been repeated several times and have al- 

 ways furnished the same result; namely, that the development of the 

 adult coloration can proceed in the absence of iodine, and can take 

 place before metamorphosis has been accomplished. Moreover, in 

 the above mentioned larvae the sex organs were not in a mature con- 

 dition; the development of the skin color is independent not only of 

 metamorphosis but also of the development of the sex organs. 



On the other hand, iodine treatment though it leads rapidly to meta- 

 morphosis does not advance the development of the skin coloration. 

 This was shown most strikingly in the Series Opaciim 1917, Wi. 

 In these larva? the iodothyrin treatment enforced metamorphosis 

 from 8 to 9 days after the beginning of the experiment; the small 

 and little developed larvse shed the skin and reduced the gills rapidly. 

 The coloration of the skin, however, which at this time was that of 

 an early larval stage, did not change at all under the influence of the 

 iodine; consequently metamorphosed animals resulted with the color 

 of young larvcC. 



Similar experiments were conducted with the species Amhystoma 

 maculatum. If the larvae were fed on earthworms and kept at 25°C., 

 the isolated yellow spots developed from the yellow network after 

 metamorphosis; if they were fed on thymus and kept at 25°C., meta- 

 morphosis was retarded, but the development of the yellow spots 

 proceeded notwithstanding. 



Evidently the development of the structures leading to the colora- 

 tion of the skin are caused by substances which are not identical with 

 the substances causing metamorphosis, nor are they identical with 

 the substances causing development of the sex organs. So far then, 

 we have found three groups of organs, each of which is controlled by 

 a different substance, or rather a different set of substances, as will 

 be discussed below. 



