260 



HISTOLOGY 



seen in parts of the mucus that covers the epithelium, as is also the case 

 in the mammals. The distal ends of the sustentacular cells form an 

 even surface. The pigment that gives the characteristic yellow or brown 

 color to this surface in so many forms of vertebrates is here deposited as 

 a thin layer in the outermost parts of these cells. The nuclei are easily 

 distinguished from the nuclei of olfactory cells by an oval form and 

 smaller size as well as a different chromatin pattern. They lie in a broad 

 zone of the epithelium composed of the distal two thirds . of the cells, 

 and are therefore to be found at varying heights in the cell. In the 



FIG. 228. Bit of olfactory epithelium from the fowl, Callus domesticus . Seen in situ to left. 

 Several individual cells shown separately to right, sup.c., supporting cells; sen.c., sen- 

 sory cells; ba.c., basal cells. 



mammals they are almost always found at one height in the cell, and 

 therefore lie in a single plane and appear as a single row in section. 



The third kind of cells in this tissue, the basal cells, are ectodermal 

 in origin, and therefore an integral part of the epithelium. In the fowl 

 this fact does not appear to advantage, the cells looking as much like 

 branching connective-tissue cells as anything else. This appearance is 

 intensified by the lack of a clearly defined basement membrane. The 

 cytoplasm of the basal cells is granular and branching and lacks definite 

 boundaries. It forms a reticulum with the cytoplasm of its neighbors, 

 and the other epithelial elements rest upon it. Iri man the basal cells lie 

 in a very much narrower row between the bases of the supporting and 

 olfactory cells. 



The olfactory cells are to be seen lying between the sustentacular 



