470 EDWARD li. POULTON. 



epithelial cells. At the same time I concluded that this method 

 of development was of comparatively recent date in Perameles, 

 while the singularly complete accesssory apparatus suggested 

 that some other and more primitive form of terminal organ had 

 not long been supplanted^ and had probably coexisted with 

 these perfect additional structures. Arguing entirely a priori 

 the suggestion was made that the primitive type of bulb was 

 papillary in position and subepithelial in structure, and had 

 gradually given way to a bulb that was interpapillary and 

 epithelial. 



At the time of this suggestion I had little hope that such a 

 primitive bulb would ever be seen. It seemed probable that 

 the stage had existed, but that it was incapable of direct proof. 



The very next tongue I worked upon — that of the highly 

 ancestral Ornithorhynchus — supplied a bulb that was at once 

 papillary and subepithelial. The new bulb, although in some 

 cases retaining its original position beneath the epithelium, has 

 usually ascended and acquired epithelial cells, and has finally 

 penetrated the surface as a gustatory pore. This latter was a 

 structure which I had not expected to appear until a later 

 stage. Nevertheless, in these new bulbs and their arrange- 

 ment we can see a cause why this should not be the permanent 

 type. 



It is obvious that a subepithelial end organ, specialised for 

 gustatory stimuli and raised until it is in actual relation with 

 the exterior through an aperture, must be extremely sensitive. 

 In fact it is probable enough that such an end organ is too 

 sensitive for the purpose, especially when continual friction 

 and the mechanical effects of accidental particles are taken 

 into account. Evidence of this is seen in the entirely unique 

 protection afforded to the more exposed anterior gustatory 

 ridges, a protection which must seriously interfere with their 

 efficacy. It is therefore probable that a less delicate form of 

 terminal organ arose, which could be brought into closer rela- 

 tions with the stimuli. This I believe was the cause of the 

 change of type, and not increased sensitiveness, except in so 

 far as this is caused by greater exposure. At the same time 



