264 



HISTOLOGY 



The distal end of the cell is specialized into a distinct rod or cell- 

 organ of smell. This rod may be clothed by a cuticular and hair-shaped 



cap, or it may be found at the bot- 

 tom of a depression or " pit " whose 

 object is to protect it. This pit is 

 formed by the surrounding hypoder- 

 mis, and is sometimes almost covered 

 over on the top. The olfactory cell 

 is sometimes very large and multi- 

 nuclear. 



The olfactory cells of the carrion 

 beetle, Necrophorus. This insect is 

 one of the first to arrive on the scene 

 when an animal dies and begins 

 to decay. The beetle has antennas 

 whose terminal joints are provided 

 with parallel plates. The surfaces of 

 these plates are lined with hypoder- 

 mal cells that have built the greater 

 part of the heavy chitinous wall. 

 Among them lie the sense cells of 

 smell, and some, perhaps, of touch 

 also. Together, in our transverse 

 section (Fig. 231), they form a very 

 thick, apparently single, layer of epi- 

 thelium, whose cells are so long and 

 narrow that their exact boundaries 

 cannot be easily determined. Occu- 

 pying the center of the plates is a 



FIG. 231. Olfactory epithelium from the w & . . 



antenna of a carrion beetle, Necrophorus. niaSS of COnnCCtlVC-tlSSUC Cells, nerve 



cu., outer layer of the elaborate cuticle; fibers, and some large and very re- 



i.cu., inner layer of the same; hy.c., hypo- i 11 11 i 



dermal cells; sen.c., sensory cells; ol.or., markable Cells whose exact meaning 



hy. c. 



nv.f. 



olfactory perceptory organs of the sensory 

 cells; nv.f., nerve fibers coming from sen- 

 sory cells; t.h., tactile hair; gl.c., gland 

 cells, whose ducts pass through the olfac- 

 tory epithelium to open through the cu- 

 ticle. X 950. 



is not clear. 



The epithelium should first be 

 studied. It shows plainly two kinds 

 of cells. The thinnest is the wall-form- 

 ing hypodermis cell, whose smaller 

 and narrower oval nucleus shows a compact granular chromatin pattern. 

 The cuticle is peculiar. It may be said to have two layers. The 

 outer layer is chitinous, but at many points it is depressed into deep pits. 

 These pits do not open altogether through to the inside, but have their 

 lower floor everted into the cavity so that it lines the pit as a second and 

 very thin wall whose entire surface is perfect. Other, and less deep, 



