32 



R. C. GESTELAND AND OTHERS 



of odors that accurately describes the selectivity of a limited number of 

 receptor types. However, judging only on the basis of strong responses, 

 it appears to us that there is a limited number of types of receptors. 



Fig. 15. The stimuli used in various experiments, the number of cells on 

 which each was tried, and the number of those which responded with an increase 

 in discharge rate are tabulated here. Weak and strong responses are lumped 



together. 



DISCUSSION 



The chemical selectivity of the olfactory receptors in the frog's mucosa 

 which we see is a curious and unsatisfying kind of selectivity. It is ana- 

 logous to a collection of poorly constructed optical filters. We could 

 describe the optical filters as follows. No two are quite alike. The res- 

 ponse spectrum changes with repeated use and environmental conditions. 

 The transmission band has many notches in it and the sides of the trans- 

 mission band fall off slowly and irregularly. Yet, the yellow ones are 

 clearly a group apart from the red ones. We do not suggest that nature 



