216 



PRINCIPLES OF ANIMAL BIOLOGY 



adult occurs gradually enough within the pupa, but the emergence of 

 the fly is sudden. These marked and more sudden modifications make up 

 complete metamorphosis. 







Fig. 188. — Incomplete metamorphosis of a bug. The most easily recognized change is the 

 gradual growth of the wings. {From Frost, "General Entomology.") 



Problems of Development. — The question naturally arises, how are all 

 these complicated developmental changes brought about? This is the 

 general problem which experimental embryologists have set themselves. 

 Some progress in solving it has been made, but much remains to be done. 

 The knowledge already gained has to do with such questions as why the 

 embryo is placed in a given position in the seemingly indifferent material 

 of the fertilized egg; how a structure is stimulated to develop, and how it 

 is guided so as to acquire its characteristic form; the time at which the fate 

 of any bit of tissue is settled, and whether the decision at that time is 

 final or revocable; whether development is a sorting out and the loss of 

 capacities, or a gain of new ones; the importance of the mere position of a 







Fig. 189. 



-Complete metamorphosis of a fly. The successive stages shown are the larva, 

 pupa and adult. {From Frost, "General Entortiolofjy.") 



piece of tissue in the embryo, in deciding what it shall become; whether 

 a tissue is passively moved about l)y some force, or actively assists in the 

 change; whether the agencies which direct development reside within 

 the colls, or impose their control from the outside; and the duration of any 

 influence in relation to the period within which it can normally be efTec- 



