278 GENERAL ANA TOM Y OR HISTOLOG Y OF BLO W-FL Y. 



may be further subdivided into mucigenic and serous cells. 

 The sensory epithelial cells are all epiblastic in origin and 

 usually chitinogenic. 



The epithelial cells of the embryo are distinguished by their 

 small size and their arrangement in several layers. As the 

 cells of these layers interlock, they cannot be termed stratified 

 in the strict sense of the term ; they may be termed tran- 

 sitional, according to the usual nomenclature, but it scarcely 

 indicates their nature. I prefer to term such epithelia 

 embryonic. In the Blow-fly, at least, this variety is only seen 

 in the embryo, in the imaginal discs, and young nymph. The 

 epithelia of the larva are remarkable for the rapid growth of 

 the individual cells, which, as already mentioned, attain extra- 

 ordinary dimensions in the adult larva. The epithelial cells of 

 the imago are very variable in size, but are usually far smaller 

 than those of the larva. 



Embryonic Epithelia. The cells are very small, rarely exceed- 

 ing 5^ to 6" in diameter, and are multiplied with great rapidity, 

 probably by indirect cell division, although, from their 

 small size, it is difficult to demonstrate nuclear figures. The 

 appearance of the nuclei is, however, indicative of karyo- 

 kinetic changes. The cells are cubical, columnar, or fusiform, 

 exhibiting one or more thin cuticular laminae on their free sur- 

 faces. They are glued together by a firm intercellular cement 

 material. 



Chitinogenic Cells (hypodermic cells). The cells which lie 

 beneath the cuticular epidermis of the skin, and those which 

 form the subcuticular layer of the stomodaeum and procto- 

 daeum, are the principal chitinogenic cells. The cells beneath 

 the cuticular skeleton are properly termed the hypoderm. They 

 are either flattened or columnar. Those of the larva have 

 already been described (p. 37). In the imago the hypoderm 

 cells are far smaller, and are either columnar or tesselated. 

 The latter are usually found under the transparent syndes- 

 moses, where no muscles are inserted. The cells into which 

 the muscles are inserted are columnar. Those at the bases of 

 the setae are spheroidal, and large in proportion to the size of 



