HISTOGENESIS IN INSECT DEVELOPMENT, AND 



CELL SPECIFICITY. 



VERNON L. KELLOGG. 



In the development (ontogeny) of insects with complete meta- 

 morphosis the imaginal antennae, mouth-parts, legs and wings 

 are produced from small buds, or histoblasts, in the larval derm. 

 These histoblasts or imaginal buds arise by the shallow or deep 

 invagination (one for each histoblast) of small regions of the 

 larval cellular skin layer including originally, in each case, only 

 comparatively few derm cells. The position of these invagina- 

 tions, and therefore the participation of derm cells in the future 

 leg or wing development, seems to be determined wholly with 

 reference to the future imaginal organ, and not at all with refer- 

 ence to any difference in degree of differentiation among the cells 

 of the larval derm. The wing-buds arise from the latero-dorsal 

 regions of the meso- and meta-thoracic segments, the leg-buds 

 from the latero-ventral regions of each of the three thoracic 

 segments. 



The larval derm is certainly not to be looked on as composed 

 of wholly undifferentiated embryonic cells. These derm cells 

 make up a definite organ, or part, of the larval body, with defini- 

 tive position and particular functions. All these cells, and no 

 others in the body, secrete 1 chitin ; some of them secrete noxious 

 fluids, ill-smelling, acrid, poisonous. Many of them, perhaps all, 

 secrete moulting fluid at the times of the regular larval moults ; 

 many are specially sensitive, many bear sense-hairs or papillae. 



The invagination and beginning development of small parts 

 of this derm, in a wingless and legless larva of a fly or honeybee 

 or of some other specialized insect, may occur in a very early lar- 

 val stage (in some cases, indeed, indications of the future histo- 

 blasts are apparent in just-hatched larvae), but in most cases the 

 invagination does not appear until a certain part of the free lar- 

 val life has been lived. That is, the larval derm has for awhile 



1 The chitin secreting capacity of the anterior and posterior thirds of the insect ali- 

 mentary canal is due to their deeply invaginated derm. 



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