A. M. Spurgin 81 



a thin layer of dentine lias been deposited and much earlier than is the 

 case with most animals. Kose (93, p. 448) describes the same condition 

 in the teeth of reptiles. The condition I find is exactly what we sliould 

 expect. As is well known, the dentine is deposited first and the outer 

 layer and stellate reticulum of the enamel organ do not disappear until 

 after the first layers of enamel have been deposited (Sudduth, 86, p. 

 640). Eose and Ballowitz, however, found no enamel and Ballowitz 

 describes the early degeneration of the entire enamel organ. 



The bud for the last tooth, which has no predecessor in the milk- 

 dentition, was considerably smaller than the other seven. A well-rounded 

 dental papilla was present, and the enamel organ was connected with 

 the enamel organ of tlie seventh tooth by an epithelial band consisting 

 of several layers of cells (Plate II, Fig. 6). I could trace this band 

 distinctly through ten or twelve sections from the longitudinal series 

 of both upper and lower jaws of the 8.5 cm. emljryo. I was also able 

 to follow it in the transverse serial sections of the lower jaw of the 

 same embryo. 



As will be seen, the results of my work on the tooth embryology of the 

 armadillo diifer in several important points from those of Eose and of 

 Ballowitz. Eose described and figured the bud for the last tooth as 

 coming from the mouth cavity direct, but it had not as yet expanded 

 into the enamel organ and no dental papilla was present. What Eose 

 had was probably a tubule of one of the glands which appear in the 

 region behind the last tooth-bud. 



As has been mentioned, Eose describes as the secretion product of 

 the enamel cells, a thin structureless membrane which lies directly 

 against the dentine and corresponds to the Xasmyth's membrane of other 

 animals. It is very evident that such a membrane does not exist 

 between the dentine and the enamel which I have shown to be deposited 

 later. Eose may have seen a very thin layer of enamel. 



I shall not enter into a discussion of the epithelial ring upon which 

 Ballowitz lays so much stress, since I did not have access to the litera- 

 ture upon the subject, but he is certainly wrong in asserting that the 

 only function of the enamel organ is to give the form to the developing 

 tooth, and to give off the epithelial ring. In regard to the presence 

 of the stellate reticulum, Eose offers no explanation. Ballowitz. while 

 recognizing that the stellate reticulum is found only in tooth -buds in 

 which a layer of enamel is afterwards deposited, denies that it has any 

 connection with the deposition of this substance. It is indeed difficult to 

 see how Ballowitz could have failed to see any significance in Tomes' 

 processes, which he described in connection with the enamel cells. He 

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