GENERAL SUMMARY 



087 



ontogeny of the insect (Fig. 636, (7), just as the germs of the diges- 

 tive canal, nervous system, and tracheae are appearing. The single 

 median thickening (y) is destined to form the ventral cephalic bud, 

 while the pair of thickenings behind (rf) become the dorsal buds, 

 those homologous with the cephalic buds of Musca. 



-cl 



FIG. 636. Imaginal buds in Musca, A, in Corethra, B, in Melophagus, C, in embryo of 

 Melophagus : dorsal view of head ; b, bud ; p, peripodal membrane ; o, cord ; hy, hypodermis ; cl, 

 cuticula ; <. stomodaeum ; . ventral cephalic bud, behind are the two dorsal cephalic buds (d) 

 After Pratt. 



The thoracic buds, which arise as hypodermic thickenings, do not 

 appear until late in embryonic life, until the time of the involution 

 of the head. 



Pratt did not observe in the embryo the buds of the internal 

 organs and of the abdominal hypodermis, and thinks it probable 

 that they appear first in the larva. 



c. General summary 



We have seen that in Coleoptera, Lepidoptera, Diptera, and 

 Hymenoptera, and with little doubt in all the holometabolous insects, 

 the parts of the imago originate in single formative cellular masses 

 (imaginal buds) already present in the larva, and often even in the 

 later embryonic stages. There are such imaginal buds for each part 

 of the body, for the appendages of the head, for the legs and wings, 

 for the ovipositor, and probably for the cercopods, for the hypo- 

 dermis of the abdomen, and for the different sections of the diges- 



