INTERRELATION OF THE CHEMICAL SENSES 183 



mal, the future olfactory region remained unchanged and 

 thus retained its original invertebrate character. This 

 region became the olfactory epithelium of the developing 

 vertebrate, the most primitive chemoreceptor in this 

 group of animals. 



The organs next in this series were the common chem- 

 ical receptors. The neurones for these organs were 

 differentiated from the neurones of the primitive inverte- 

 brate skin by a central migration of their cell-bodies till 

 they became part of the spinal ganglia and thus left in the 

 integument free-nerve terminations as receptors. This 

 type of chemoreceptor is found generally in the skin of 

 fishes and amphibians and in the mouths, nasal chambers 

 and other moist cavities of the air-inhabiting vertebrates. 



The third and last type of the vertebrate chemorecep- 

 tor is the gustatory organ. In this type the conducting 

 neurone presents exactly the condition met with in the 

 common chemical receptor excepting that its nerve ter- 

 minals, instead of being free in the integument, are asso- 

 ciated with epithelial taste-buds. This type of receptor 

 was probably derived from the second type by the appro- 

 priation of taste-cells from the integumentary epithelium. 

 Thus the three types of vertebrate chemoreceptors 

 appear to be genetically related in that the olfactory 

 organs represent what may be called the first generation, 

 the common chemical the second, and the gustatory the 

 third (Parker, 1912). 



But within each type much detailed differentiation 

 has taken place. It seems to be quite impossible to ex- 

 plain the variety of olfactory sensations without assum- 

 ing a differentiation among the receptors of the olfactory 

 field. In the common chemical sense the receptors on 



