EDUARD UHLENHUTH 481 



they would be if kept at high temperature. We have observed this 

 phenomenon frequently and in looking for an explanation have tried 

 to find a relation between the size of the larvae and the age at the 

 time of metamorphosis, similar to that existing between the latter 

 quantity and the rate of growth. Such a relation, however, does 

 not exist. 



But if we assume the action of an excretor substance in metamor- 

 phosis, the phenomenon in question can be readily explained. Com- 

 paring R X A for the first worm-fed series (C 1916) in Table I, 

 which was kept at a temperature of 10°C. below that of the other 

 series, with the rest of the series of Table I, we observe that it is 

 very high (10 per cent) above the average, which would indicate that 

 metamorphosis in this series was more retarded than the correspond- 

 ing rate of growth would demand. Since in this series a drop of the 

 growth curve similar to the drop of the growth curve of the thymus- 

 fed animals did not occur, this case of undue retardation of metamor- 

 phosis cannot be explained in the same way as in the thymus-fed larvae, 

 i.e. by asuming that from a lack of iodine in the thyroid gland a de- 

 structive compound was excreted by the thyroid. The only way to 

 explain this case of retardation is to assume that at low temperature 

 less excretor substance is evolved than at high temperature during 

 an equal rate of growth processes. That this should be possible is 

 not at all surprising, but was to be expected since it is well known 

 that the temperature coefficients for different physiological processes 

 may differ greatly. Loeb, for instance, pointed out that not only the 

 temperature coefiicients but also their variations at the lower and 

 upper temperature scale differ considerably in different physiological 

 processes.^ 



But if the amount of excretor substance produced by an equal rate 

 of growth is less at low^ temperature than at high temperature, the 

 animals kept at low temperature must grow longer at an equal rate 

 than those kept at high temperature before that amount of excretor 

 substance is produced which is required to bring about thyroid ex- 

 cretion. Consequently the low temperature larvee must reach a larger 

 size than the high temperature larvae, before they can metamorphose. 



^Loeb, J., Mechanistic conception of life, Chicago, 1912, 212. 



