88 COMPARATIVE ANATOMY 



iodine, and the granules may easily be stained with this 

 reagent in fresh liver-cells. 



In certain parts of the body individual epithelial cells are 

 modified in connection with sensory surfaces. Such modified 

 epithelia are known as sensory or neuro-epithelia, and 

 the cells composing them may conveniently be described as 

 end-cells. They are found in the frog in the forms of taste- 

 cells on the tongue and palate, olfactory cells in the olfactory 

 region of the nasal cavities, rods and cones in the retina of 

 the eye, hair-cells in the labyrinth of the ear, especially in 

 patches on the ampullae of the semi-circular canals, and touch- 

 cells on the touch-spots of Merkel, which are best seen on the 

 swelling on the thumb of the male during the breeding season. 

 There are also end-cells in the sense organs of the lateral 

 line, highly developed in the tadpole, but degenerate in the 

 adult frog. 



We may take the olfactory cell as a good example of a neuro- 

 epithelial cell (fig. 15, D}. In the olfactory region of the 

 nasal chamber the epithelium is mostly composed of columnar 

 epithelial cells, produced internally into long branched pro- 

 cesses. Amongst these are placed cells, each of which possess 

 an ovoid body enclosing a large nucleus. From the body 

 a peripheral or distal and a central or proximal process is 

 given off. The former reaches the surface of the surrounding 

 epithelium, and bears at its end a few (5 to 8) stiff hair-like 

 processes, which may be modified or fused cilia, but, unlike 

 cilia, are immovable. The central process of the cell consists 

 of a very fine varicose fibre, which passes into a sub-epithelial 

 plexus composed of similar fibres from other olfactory cells 

 and the inner branched ends of the epithelial cells. Each 

 fibril of an olfactory cell is continued into a fibril of the 

 olfactory nerve. 



The rods and cones of the retina are peculiar structures, 

 each being a product of rather than a metamorphosed 

 epithelial cell. A rod is a cylindrical rod-shaped body 

 about 0*05 mm. in length, consisting of two parts, a longer, 

 highly refractive, outer limb, and a shorter, more homogeneous, 

 less refractive, inner limb, separated by a small plano-convex 

 structure known as the lenticular body (fig. 15, C). At the 

 base of each rod is an oval nucleus surrounded by a very thin 

 sheath of granular protoplasmic matter which is continued inter- 



