478 THYROID, METAMORPHOSIS, AND GROWTH 



substance would be the toxic substance tryptophane instead of the 

 normal hormone. 



This expectation is actually fulfilled in the thymus-fed larvae of 

 Amhy stoma opacum, as is shown by Curve B, Fig. 1. In this 

 curve is represented the growth of a series of larvae of the same age 

 and from the same mother as the larvae used for the experiment plotted 

 in Curve A; both series were kept under the same conditions, but while 

 Series A was fed on earthworms, Series B was fed on thymus. The 

 thymus-fed larvae grew normally in the beginning — and in this par- 

 ticular series even more quickly than the controls, a fact explained 

 by the writer in a previous paper.^ At the 13th week we observe a 

 sharp drop in Curve B and hereafter growth was stopped and never 

 resumed again. Exactly' the same results were obtained for the 

 thymus-fed larvae of Amhystoma viaadatum and Ambystoma tigrinnm. 



The time when the drop of the curve takes place in the thymus-fed 

 animals is that at which metamorphosis should have occurred if 

 they had received norm^al food. This was not only suggested by 

 the general appearance of these larvae and by the fact that a few 

 larvae actually did metamorphose (in Series B 1916 only one 

 larva metamorphosed at this time), but it can be proved if we 

 calculate the time of metamorphosis from the value of the 

 product R X A for the worm-fed larvae. This has been done 

 in Table II for eight thymus-fed series of Ambystoma opacum. 

 Again the product R X A was calculated from the duration 

 of the larval period and the rate of growth, the latter value being 

 obtained as in the worm-fed larvae from the growth curve up 

 to the point where the drop occurred. First we notice that the 

 product R X A (Table II) in the first three series is far above the 

 average value of R X A as obtained for the worm-fed series. 

 This is due to the fact that the larvae pf these series did not meta- 

 morphose at a time proportional to their rate of growth but much 

 later, due to the absence of iodine at this time. The time at which 

 they should have metamorphosed if iodine had been present in the 

 food in a normal amount, can be calculated, however, from the pro- 

 duct R X A in the worm-fed series, which is 60, and the rate of 



« Uhlenhuth, E., Proc. Soc. Exp. Biol, and Med., 1917-18, .xv, 37. 



