EPITHELIA. 281 



tnay even exhibit lines vertical to its surfaces, corresponding 

 with the divisions between the subjacent cells. 



Whether the cuticular layers of the skin, the dermal skeleton, 

 is to be considered, as Leydig held, as an indurated exudation 

 from the cells, or as a modification of the cell-substance, is a 

 question which is no longer open to doubt. The direct con- 

 version of the epiblastic cells of the nymph into cuticular tissue 

 is perfectly obvious (see Development of the Nymph), and 

 I suspect that the laminated endostracoid layers are also 

 formed in the same way, as well as the internal cuticular 

 membranes. 



This view is further supported by the gradual thinning of 

 the hypodermal cells as the cuticular layers are developed, and 

 by the eventual disappearance of the hypoderm beneath the 

 harder sclerites of the adult imago ; by the manifest fusion of 

 the bases of the cells and the delamination of the basement 

 cuticle of the sericteria of the larva ; by the sculpturing of the 

 surface of the epidermis, and by the presence of minute and 

 often complex setae on its surface. 



The formation of the branching setee on the surface of the 

 membranous parts of the proboscis is quite inexplicable on the 

 hypothesis that the cuticular layers are formed by a fluid or 

 semi-fluid excretion from the cells. The laminated structure 

 of the cuticle is, it is true, sometimes unbroken by any 

 indication of areas corresponding with those of the sub- 

 jacent cells, but this is sufficiently explained by the manifest 

 fusion of their bases, which precedes the process of chitini- 

 zation. 



Leydig regarded the epidermis as a fibrillated matrix, but, as 

 far as I know, there are no indications of fibrillation in the 

 dermal cuticle of Insects. In the Crayfish, however, the 

 hypodermis is said to be fibrillated, but this fact is no argument 

 against its cellular origin, as a similar fibrillation occurs in 

 the wall of the proventriculus of the imago of the Blow-fly, 

 and is undoubtedly the result of the fibrillation of the extremities 

 of the epithelial cells of which it is composed. 



In the larvae of some Diptera the cuticle is divided distinctly 



19 2 



