178 H. SPENCER HA BET SON. 



teeth are borne on this portion (fig-. 16, e. o.f. ; this is the 

 eleventh), as is shown in the figure. The twelfth is only 

 indicated in the section drawn by a slight swelling anterior 

 to the eleventh. 



In number and size the palatine and mandibular teeth 

 show no special features, and correspond with those of the 

 skull described. The mandibular dental lamina (fig. 16, d. I.) 

 has a free posterior continuation, as in earlier stages, and this 

 bears a small, slightly calcified tooth and the enamel organ of 

 another (neither shown in fig. 16). 



The most interesting observation I have made in this 

 embryo relates to the vomerine dentition. I have shown in 

 the historical portion of this paper that the occurrence of 

 vomerine teeth is comparatively rare, and that considerable 

 variation and inconstancy obtains in their relative size and 

 even in their number. 



In the present embryo I find on the left side, very near the 

 middle line, a longitudinal thickening of the epithelium of the 

 palate, situated below the posterior portion of the vomer. 

 Indenting the middle portion of this thickening is a meso- 

 dermal papilla and a well-marked early enamel organ formed 

 from the overlying epithelium (fig. 13, V. e. o.). Between 

 this structure and the bone there is a dense mass of flattened 

 mesodermal cells. In a corresponding position on the right 

 side is a slight epidermal thickening, but only a faint though 

 unmistakable indication of an enamel organ and papilla. 

 The condition on this side is, I believe, degenerate as com- 

 pared with the other, and it is not a case of later deve- 

 lopment. 



Returning to the teeth of the future functional series in 

 the upper and lower jaw, we find that the labial and lingual 

 epidermal ingrowths are now very conspicuous structures, 

 and the former at least is beginning to form large cavities 

 and to show a lobed character, indicative of the future 

 glandular structure of a portion of its constituent cells. The 

 cavities ai-e the first stages in the formation of the labio-dental 

 groove, which is found in the next embryo (fig. 14, Ld. gr.). 



