HISTOGENESIS IN INSECT DEVELOPMENT. 121 



subserved only, and has subserved fully, the functions of skin, and 

 its cells have certainly attained whatever differentiation they need 

 for the performance of these functions. It is far and away a long 

 cry back to the embryonic, undifferentiated, the non-specific con- 

 dition. But apparently any part or region of this derm which 

 may, by its position, be the region or part needed to develop an 

 antenna, a wing, a leg, male clasper, female ovipositor, or a sting, 

 can respond to the need, and by invagination (for protection's 

 sake), rapid growth and proliferation of cells, quick differentiation 

 and arrangement, and final evagination (at the time of the last lar- 

 val moulting, /. e., pupation) produce the needed organ. This 

 organ may be tubular and segmented, and the segments may be 

 similar (antennae) ; or dissimilar (leg) ; or it may be a great flat- 

 tened sac, supported by tubular skeletal ribs and covered by a 

 million and more tiny other striated, pigment-bearing, flattened 

 sacs (the butterfly's wing with its scales) ; or it may be the ex- 

 quisite mechanism of the bee's sting. 



This histogenesis of the imaginal parts of the fly, the bee, and 

 the moth is, to my mind, an extremely suggestive phenomenon 

 when considered in the light of its relation to the theories of cell- 

 specificity or cell-non-specificity. Quite as positively as the 

 more familiar cases of restorative regeneration (legs, eye-lenses, 

 tails and what not of various vertebrates), does this radical histo- 

 genesis, common to the ontogeny of all insects with complete 

 metamorphosis, make it impossible to limit the germ-plasm to the 

 germ-cells. It stands strongly opposed to any theory of abso- 

 lute cell-differentiation or cell-specificity. 



STANFORD UNIVERSITY, CAI.IF. 



