474 THYROID, METAMORPHOSIS, AND GROWTH 



bringing about metamorphosis. If, however, a thyroid gland is 

 present, all or most iodine taken up by the organism is retained and 

 stored up in the thyroid. At a certain time during the life of the 

 larva the thyroid suddenly begins to excrete the iodine stored up 

 during the larval period, and metamorphosis results. 



From this it is evident that under normal conditions the iodine is 

 not the only substance needed in metamorphosis; there must be still 

 another substance which, when present in a certain quantity, causes 

 the thyroid to excrete the thyroid hormone. The experiments to be 

 reported in this article not only furnish evidence of the actual exist-, 

 ence of such an excretor substance, but they also demonstrate that 

 it is evolved during the processes which lead to the growth of the 

 organism. 



Metamorphosis and Rate of Growth in Worm-Fed LarvcB. 



In eleven series of larvse of the species Amhystoma opacum the rate 

 of growth during the larval period of active growth has been deter- 

 mined in the following way. 



In each series the time of metamorphosis was recorded for each 

 single individual; this can be done very accurately in this species if the 

 first shedding of the skin and the reduction of the gills to mere stumps 

 without fringes are taken as the indication of metamorphosis, both 

 phenomena occurring within a day. That these two processes are 

 actually controlled by the influence of the thyroid, in contradistinc- 

 tion to many other processes, has been indicated already in former 

 articles^ and will be shown in detail in a later publication. From 

 the values obtained in this way the average length of time of the 

 larval period was calculated for each series (Table I). 



Each individual was measured once a week and the average sizes 

 obtained from those values for each series were plotted in curves. At 

 the time of metamorphosis a sharp drop of the curve takes place, 

 due to a discontinuation of growth, and even a diminution of the size 

 of the animals which may last for one or several weeks before growth 

 is resumed. In Series A 1916, the curve of which is shown in Fig. 1, 



^'Uhlenhuth, E., /. Exp. Zool., 1917-18, xxiv, 237; 1918, xxv, 135; /. Gen. 

 Physiol, 1918-19, i, 305. 



