EXISTENCE OF AN ENAMEL ORGAN IN AN ARMADILLO. 47 



reticulum, wliich seen by the naked eye looks almost semi-fluid 



or gelatinous. When the deposition of enamel is completed 

 this gelatinous mass disappears and the external comes into 

 contact with the internal epithelium ; a fact of some interest 

 in com. -tion with the dental germs of the armadillo. 



From the line of cells which connect the enamel organ with 

 the deep layer of the oral epithelium (by an inflection of 

 which it was originally formed) runs downwards a process 

 consisting of a double row of cells, not unlike a simple tubular 

 gland ; this has been shown by Professor Kolliker to be the 

 enamel germ of the permanent tooth. 



The earliest condition of dental germ which I have 

 observed in the armadillo is represented in fig. 2 ; the 

 section was taken from a foetus about an inch long. At c is 

 seen the enamel germ, consisting of a mass of cells, those 

 upon its surface being distinctly differentiated from those in 

 its interior, and forming a sort of epithelial investment to it; 

 it is connected with the epithelium of the oral cavity by the 

 usual thin neck of cells, and it is similar to, and in fact per- 

 fectly indistinguishable from, the enamel germs of those other 

 mammals which do have enamel upon their perfected teeth. 



The future dentine germ and its processes forming the 

 capsule are as yet only represented by a slight opacity, which 

 has been rather exaggerated in the draAving. 



The next stage whicli I have observed is represented in 

 figs. 3 and 4 ; the dentine papilla (b) has taken a definite 

 form ; the cells on its exterior have become differentiated to 

 form an odontoblast layer, and towards the top a thin cap of 

 dentine (h) has already been formed. 



Closely applied to the formed dentine, and extending below 

 it on the surface of the dentine germ, is an investment of 

 columnar cells, exactly like the internal epithelium of the 

 enamel organ (enamel cells) of other mammals, and at first 

 sight this might appear to be all that there is. But in speci- 

 mens which have been slightly displaced or torn, the epithelium 

 is seen to really consist of two layers, one of which adheres 

 closely to the dentine, and the other to the capsule ; and to- 

 wards the base of the dentine germ, where the epithelial layer 

 is reflected over it, it can always be seen to be double (see y, 

 figs. 3 and 4) . In fact this apparently single layer is really 

 made up of the internal and external epithelia in close con- 

 tact with one another, owing to the absence of that reticu- 

 late tissue wliich separates them in the enamel organ of 

 those animals which form enamel on their teeth. And this 

 is just what has already been noted to be the condition of 

 all enamel organs after the completion of the enamel. 



