TONGUE OF PERAMELES NASUTA. 79 



taneously the nerve-supply would be increasing in amount and 

 advancing in complexity. 



The chief taste areas will have also been sheltered by 

 becoming enclosed in folds, either of the circumvallate or 

 foliate type. Of course the exact order of these events cannot 

 be made out, nor is it of great importance. The important 

 point is that a time must have come when a more specialised 

 form of terminal organ, coming into closer relations with the 

 stimulus, was substituted for one of a more general type. It 

 is not necessary or possible to exactly define this point of time 

 relatively to that of the advancing accessory structures, but, as 

 before mentioned, the high place reached by these in the cir- 

 cumvallate papillae, accompanied by a low type of bulb, renders 

 it probable that the latter was subsequently developed. It is 

 also noteworthy that these accessory advantages may have 

 been more necessary with a less advanced type of terminal 

 organ. 



This substitution of a higher form of terminal organ seems 

 to have taken place by the growth of the columnar cells forming 

 the lowest layer of an interpapillary process. In this way the 

 cells would approach the surface, converging as they elongated. 

 There would also be a gradual concentration of the nervous 

 elements upon these new end organs and a corresponding 

 withdrawal from the papillary structures. 



It seems to me that this stage is reached in the fungiform 

 papilla (fig. 3) above described. The deeply placed columnar 

 cells of the interpapillary process have elongated, and so come 

 into closer relation with the surface than the less deeply placed 

 cells of tlie papillary process ; that is to say, the former would 

 be more advantageous as terminal organs. This account of 

 the origin of taste bulbs explains one important difference be- 

 tween them and the other structurally related end organs, as 

 those of the olfactory region, or sacculi and ampullae, i.e. in 

 the fact that the gustatory cells are massed together in little 

 groups surrounded by protective cells, while the auditory cells 

 in the positions above mentioned and the olfactory cells are 

 isolated, each being separately protected by columnar cells. 



