156 J. T. WILSON AND J. P. HILL. 



in the older specimen, has taken shape as the small and still 

 nncalcified enamel organ shown in text-fig. 2 as " v," in front 

 of the calcified tooth '^ w." 



The view of the homology of the small vestigial tooth "dv" 

 of the younger specimen with the nodular series of the older 

 is further strengthened by the fact that its lower opponent, 

 " dv/' is in a more advanced stage of involution than upper 

 '^ dv." It lacks the definite dentinal character of the latter, 

 its structurpJ constituents show concentric lamination, and 

 altogether its appearance is strongly suggestive of the 

 structure of the nodules of the older stage. 



(5) Origin of the nodules. — It might have been ex- 

 pected that the possession of a younger stage for comparison 

 with that represented by our specimen "Beta" would have 

 enabled us to elucidate the mode of origin, or at least some 

 definite pai't of the early history of the nodular vestiges, since 

 these are obviously in process of involution in the older stage. 

 The two stages, however, prove to be too remote from one 

 another to yield positive and conclusive evidence as to the 

 mode of origin of the nodules. A priori one might have 

 expected to find the vestigial teeth represented in the younger 

 specimen by precocious enamel-organs. But although the 



large permanent teeth t/ n have not yet come into existence 



"w" "x" 



as distinct enamel-organs, and both 77 — y, and 77 — j-, are in the 



condition of well-developed enamel-organs, there are no 

 definite enamel-organs to represent the early phase of the 

 nodular vestiges of the later stage. 



Nevertheless we find that there are present, at intervals 

 along the labial aspect of the dental lamina, a succession of 

 peculiar structural differentiations which there is small room 

 for doubting to be the beginnings of the nodular structures. 

 These structural differentiations are of the nature of a series 

 of invasions or deep indentations of the neck of the dental 

 lamina, on its labial aspect, near the level of its continuity 

 with the deep surface of the mouth-epithelium. They occur 



