232 SMELL, TASTE, AND CHEMICAL SENSE IN VERTEBRATES. 



is also met with among the invertebrates. Thus Pollock (1883) has shown and 

 I have verified his observations, that when a piece of food is put in a natural 

 pool of sea-water in which there are actinians, they respond to its presence bv 

 expanding and reaching out their tentacles even when the ford is at some distance 

 from them. The receptors for this response are ectodermic cells almost identical 

 with those in the olfactory epithelium of vertebrates. I, therefore, believe that 

 the primitive chemical sense of the lower animals is more nearly reproduced in 

 the olfactory sense of the vertebrates than in their common chemical sense or 

 their sense of taste and that Pollock was not unjustified in speaking of a sense of 

 smell in actinians. This primitive chemical sense was, in my opinion, inherited 

 by the ancestors of the vertebrates probably as a common integumentary sense 

 and though now transformed over much of its original extent into another 

 sensory mechanism, it has not been entirely lost but is still retained with many of 

 its primitive characteristics in the vertebrate olfactory epithelium. 



The organs into which the primitive chemical-sense receptors of the ancestral 

 vertebrate were transformed are in my opinion the common chemical sense 

 organs such as are found now in the epidermis of most water-inhabiting verte- 

 brates. This transformation was accomplished by the migration of the cell-body 

 of the primitive sensory neurone from its peripheral position in the integumentary 

 epithelium to a deep-seated one near the central nervous organ (Retzius, 1892), a 

 migration which probably occurred under the influence of increasing metabolism 

 at the proximal end of the neurone as the central nervous organ grew in com- 

 plexity, and which may have resulted in a reduction of the sensitiveness of the 

 distal end of the neurone in consequence of the withdrawal of the metabolic 

 organ. The distal end, however, by forming a rich arborization among the 

 cells of the thickening epidermis probably made good some of this loss and 

 established itself as a fairly sensitive end-organ in a rich cellular environment, 

 the epidermis. It is quite probable, as Botezat (1910) has suggested, that the 

 real stimulus for this type of nerve ending may come quite as much from the 

 activities of the epidermal cells as from the direct action of the external solution 

 itself. In other words, the external solution may so influence the epidermal 

 cells that the resulting secretions from them may come to be more important as a 

 means of stimulating the true nervous end-organ than the external solution 

 itself, i. e., these nerve terminals may exhibit a form of indirect stimulation. 



In the region of the mouth of the evolving vertebrate, these arborizations 

 of the receptors for the common chemical sense probably became associated with 

 particular groups of especially active epidermal cells which thus constituted the 

 beginnings of a taste bud and established a set of peripheral secondary receptors 

 whose sensitiveness was high because their metabolic efficiency, depending upon 

 the nearness of their nuclei, was undiminished. Thus the peripheral sensitive- 

 ness which was reduced by the distal migration of the cell-body of the primitive 

 sensory neurone may have been revived by the appropriation from the epidermis 

 of certain secondary sensory cells whose secretory activity under the influence 



