Metamorphosis 



653 



Degree of 

 Development 



DAYS 



swr.L. 



Fig. 229. Diagrammatic representation of rate of tissue changes during normal metamorphosis in 

 tadpole. Note that each structure has its own time for beginning and completion of its metamorphic 

 change. A.C.P., Resorption of anal canal piece; S.W.F.L., skin window for the forelegs; E, emergence 

 of forelegs, arbitrarily designated as day number 30. 



metamorphic and larval molts in insects 

 would appear to depend upon a qualitative 

 difference in the stimulus, the presence or 

 absence of the C.A. factor. In the interpre- 

 tation of Wigglesworth ('40, '48) this factor 

 serves to activate a different cellular mech- 

 anism than does the metamorphic hormone. 

 Because of the more rapid response of the 

 tissues to C.A. hormone the activation of the 

 metamorphic cellular mechanism is fore- 

 stalled. Hence Wigglesworth's term "juvenile 

 hormone," indicating a positive mode of ac- 

 tion, replacing his original "inhibitory hor- 

 mone" for the C.A. factor. It wovild appear, 

 however, that the evidence for this mode of ac- 

 tion is not clear-cut and an action of the C.A. 

 hormone on the prothoracic gland hormone 

 rather than directly on the tissvies has not 

 been excluded. The possibility exists that, 



at least for some organs, quantitative differ- 

 ences in the degree of stimulation of the 

 same cellular mechanism may account for 

 the different developmental changes in molt- 

 ing, pupation and metamorphosis. 



THE ACTIVATION OF THE METAMORPHIC 

 MECHANISM 



In amphibians, metamorphosis depends 

 upon a chain of events precipitated by the 

 action of the thyrotrophic hormone of the 

 anterior pituitary. In insects there is an 

 analogous situation in the action of the brain 

 in stimulating the prothoracic glands. In 

 this section it is proposed to examine what 

 is known of the factors determining the ac- 

 tivation of the initiating stimulus from the 

 pituitary or brain. 



