80 EDWARD B. POULTON. 



This difference, it appears, is simply due to the latter elongat- 

 ing from a tolerably plane surface, while the gustatory cells 

 have elongated from the curved surface of an interpapillary 

 process — approximately the segment of a sphere — and therefore 

 have met and penetrated the surface in a group. In the further 

 development of the bulbs the external columnar cells would 

 become protective, the axial cells alone acting as end organs ; 

 the columnar cells would converge to form a basal pole, as the 

 nerve supply was limited to a small area in this region (being 

 connected only with the axial cells). In fig. 3 the basal con- 

 vergence has not commenced, and in the circumvallate bulbs 

 (fig. 2) it is not nearly as complete as in higher animals. In 

 the former case I do not think that there is yet any trace of a 

 division into protective gustatory cells in the bulb, and in the 

 circumvallate papillae I am sure that the difference is not well 

 marked, even if begun. Finally, the papillary elevations would 

 disappear between the bulbs, and the latter would rest in the 

 cavities of an epithelium with a nearly plane surface below. 

 This, which is reached in the higher animals, is apparently 

 never attained in the tongue of Perameles nasuta. The 

 papillary upgrowths separating the bulbs give to them the ap- 

 pearance of interpapillary processes to a marked degree. In 

 fact, there is the greatest resemblance between the bulbs and 

 the interpapillary processes of the epithelium on the outer wall 

 of the trench — a resemblance so great as to suggest this expla- 

 nation of the origin of bulbs. 



And yet indications of the ultimate disappearance of the 

 papillae between the bulbs are seen in the fact that the papillae 

 between the lowest tier of bulbs and the ordinary epithelium 

 below, are always far more marked than those between the 

 bulbs themselves (see fig. 2). I think that a comparison of 

 figs. 2 and 3 with the figures of taste-bulbs in ' Strieker's Hand- 

 book,' by Engelmann, or in Klein's 'Atlas,' will at once suggest 

 an explanation of their origin similar to that which I have 

 given above. I must express my thanks to Mr. W. H. Jack- 

 son, M.A., for kind help and suggestions in working out the 

 above theory. 



