28o GENERAL ANATOMY OR HISTOLOGY OF BLOW-FLY. 



lation, forming a kind of tendon. Tiiis fibrillation commences 

 next the muscle, and eventually nothing is left of the original 

 cell but a stellate core of protoplasm enclosing the nucleus, 

 surrounded by tendinous fibres. 



A similar transformation of the cells of an epithelium into 

 fibrous tissue occurs in the proventriculus, but the fibres are 

 much branched, and form a close meshwork comparable with 

 that of the elastic cartilage of Vertebrates, except in the 

 extreme fineness of the fibres, which more closely resemble 

 those of white fibro-cartilage (see Proventriculus). 



Development of the Integument of the Imago. — The epiblast of 

 the nnaginal discs is differentiated into two layers, a superficial 

 one of small and a deep one of large cells. The larger epiblast 

 cells from which the setae are developed remain as the hypoderm, 

 but the great majority of the cells of the superficial layer 

 become chitinized throughout their entire substance, and ulti- 

 mately so intimately united that all trace of their original limits 

 is lost. They are transformed into cuticle, and form the super- 

 ficial part of the exo-skeleton. As earl}^ as 1870 I pointed out 

 that the epiostracoid layer, which I then termed the protoderm, 

 is cellular in character. From more recent investigations, I 

 ha\e now no doubt I was correct, for the examination of a 

 series of nymphs at the stage of development in which the 

 setae are being formed cannot fail to convince the observer 

 that the cuticular external layer is formed from the epiblastic 

 cells, not by the secretion of cuticular lamellae, but by the 

 actual conversion of the cells into a skeletal epidermis, under 

 which a second la}er of cells is developed, the true hypo- 

 dermis of the imago, chiefly by the extension of the trichogenic 

 cells of the epiblast beneath the smaller cells of the outer 

 layer. 



The Cuticular Structures. — The cuticular epidermis of the 

 imago does not differ materially from that of the larva (see 

 p. 9). Where the sclerites are dense, however, the laminated 

 structure is less apparent or entirely absent. Such parts are 

 also opaque and deeply coloured. Wherever the cuticular 

 integument remains transparent, it is distinctly laminated, and 



