652 



Metamorphosis 



that in Anura the thyroid gland at the begin- 

 ning of metamorphosis is only slightly active 

 but that through the prometamorphic period 

 becomes more and more active until in the 

 metamorphic climax it reaches a peak of 

 activity accompanied by release of some of 



Insects. No direct analysis of the problem 

 of the correlation of metamorphic events is 

 available in insects. Wigglesworth's interpre- 

 tation is that the same hormone is responsible 

 for the metamorphic molt and for the growth 

 of imaginal structures in Rhodnius. This is 



DaLjs 



Fig. 228. Metamorphic pattern and thyroxine concentration in the tadpole. Graphs show hind leg 

 growth. The times of occurrence of ten metamorphic events are shown by numbers. The top curve is 

 that of a normal tadpole. The other curves show metamorphic pattern in thyroidectomized tadpoles 

 treated with various thyroxine solutions. The number with each curve expresses the dilution of the 

 solution exponentially, i.e., 6 equals one part thyroxine in 10^ parts of water. Bottom graph of animal 

 treated with increasing thyroxine concentrations starting from 9 and going to 6-2/3. (After Etkin, '34.) 



its stored hormone. The relatively low thy- 

 roid hormone concentration in the blood at 

 the beginning of metamorphosis serves to 

 activate the very sensitive leg primordia; 

 the higher concentrations as they are built 

 up in the blood speed the metamorphic proc- 

 ess until in the metamorphic climax the 

 flooding of the body with thyroid hormone 

 produces the rapid reaction even in the least 

 sensitive tissues, such as tail and tympanic 

 ring. 



also the current interpretation of Williams 

 ('52). Hence it is possible that the temporal 

 relationships of events in pupation and meta- 

 morphosis are a function of the interplay of 

 differential tissue sensitivity and changing 

 hormone level as in amphibian metamorpho- 

 sis. In Platysamia, for example, a pulse of low 

 hormone activity leads to pupation. Then, 

 after diapause a stronger pulse leads to com- 

 pletion of imaginal development. 



On the other hand, the difference between 



