324 



THE BIOLOGY OF THE FROG 



CHAP. 



are very much elongated and cylindrical inform ; (2) basal 

 cells (Krause), which are of stellate form and lie next to the 

 connective tissue ; and (3) the ol- 

 factory cells. The latter, which 

 represent the true sensory cells, 

 are very variable in form. They 

 are usually very long and narrow, 

 much swollen out where the nu- 

 cleus occurs, and furnished at the 

 outer free end with a tuft of fine 

 cilia. At their inner ends they are 

 drawn out into a fine process which 

 is connected with the olfactory 

 nerve. 



Knauer found that ill-smelling 

 substances held near the nostrils 

 caused the frog to turn away the 

 head, but whether the olfactory 

 sense plays an important role in 

 the life of the animal is not known. 

 The Eyes. — The eyes of the 

 frog are nearly spherical in form, 

 and they are lodged in large cavi- 

 ties in the sides of the head called 

 the orbits, within which they are 

 freely movable. The outer or ex- 

 posed portion of the eye is cov- 

 ered by a transparent membrane 

 known as the cornea; the remain- 

 ing portion, which constitutes about 

 two thirds of the surface, is formed 

 by the sclerotic coat. This is an opaque, white covering 

 mainly inclosed within the orbit ; it is composed of an 



Fig. 88. — Cells of the olfac- 

 tory epithelium. On the left 

 two sensory cells with cili- 

 ated outer ends, enlarged 

 nuclei, and fine basal fibers. 

 On the right two interstitial 

 cells. (After Dogiel.) 



