672 BERTHA E. MARTIN 



In the case of the first three teeth of the lower jaw, it is evident 

 that enamel secretion has almost ceased. The vestigial milk 

 teeth of Perameles and the degenerate milk incisors of the mouse 

 have been described as consisting of a dentine cap upon which 

 no traces of enamel can be found. In Ornithorhynchus, Wilson 

 and Hill find a 'nodule' which has a connective tissue pulp, in- 

 closed by a distinct ring of dentine, outside of which is a layer 

 of columnar cells representing the inner enamel epithelium. The 

 authors consider this nodule to be a vestigial tooth. It prob- 

 able represents a more extreme case of degeneration than that 

 occurring in these front teeth of the armadillo. In these cases, 

 the authors have made no statement as to whether or not the 

 dentine secreted is normal dentine. Another sign of extreme 

 degeneration of the first three of these functionless front teeth is 

 the fact that while the fourth, fifth, and sixth teeth increase in 

 size during embryonic life, the first, second, and third teeth 

 actually decrease in size so that at birth these tooth germs are 

 smaller than in the 78 mm. embryo. Furthermore, after the 

 78 mm. stage, the first three teeth recede into the substance of 

 the jaw, this process being more marked from the first tooth 

 backwards. On the contrary, teeth four and five remain about 

 the same distance from the surface, while the sixth tooth ad- 

 vances toward the surface. 



The rudimentary front teeth probably belong to the milk 

 dentition. There is no evidence that they ever have prede- 

 cessors. I have never been able to identify any secondary tooth 

 buds in connection with these front teeth. In their vicinity 

 there are always more or less scattered groups of epithelial cells, 

 which represent the remains of the dental lamina. Sometimes 

 these groups bear a strong resemblance to secondary tooth buds. 

 Rose has described in connection with his two rudimentary in- 

 cisors lingual outgrowths of the enamel organ which he considers 

 as representing secondary tooth buds. I have found that 

 groups of epithelial cells resembling secondary tooth buds may 

 occur in any relation to the enamel organ of these teeth, and for 

 this reason, I have been led to believe that when such structures 

 occur on the lingual side they represent only a chance grouping 



