THE DEVELOPMENT OF THE TEETH. 129 



its elevated borders being formed by an increase in the 

 thickness of the layer of epithelial cells ; while in trans- 

 verse sections the proliferation of epithelial cells is found to 

 have been even more energetic in a direction downwards 

 into the substance of the jaw than it is upwards, so that a 

 cul-de-sac of epithelium dips into the embryonic sub-mucous 

 tissue. 1 



In a certain sense, then, there is a dental groove, but it 

 is not the same thing as that described as such in the text- 

 books, and it therefore better to abstain from applying that 

 or any other name to the shallow furrow of which we are 

 now speaking, which is almost filled up with spherical or 

 flattened cells, the deepest layer being, however, columnar 

 cells. From the bottom, or the side near the bottom of the 

 depression, an inflection of epithelial cells takes its origin, 

 which does not dip downwards vertically, but inclines in- 

 wards. This secondary narrow inflection of epithelium, 

 which in section closely resembles a tubular gland, consti- 

 tutes the rudiment of the future enamel organ ; a prolife- 

 ration of the cells at its deepest extremity speedily takes 

 place, so that it expands, attaining somewhat the form of a 

 Florence flask (Fig. 64). It should, however, be noted, that 

 while the inflection of epithelium takes place around the 

 entire circumference of the jaw, so that that which appears 

 in sections like a tubular gland is really a continuous sheet 

 or lamina of epithelium, the dilatations of its extremity, 

 which I have compared to a Florence flask, occur only at the 

 several points where teeth will ultimately be developed. 



The cells upon the periphery are columnar, polygonal 

 cells occupying the central area of the enlargement. Very 



(!) The epithelium having been removed by maceration or by keeping a 

 specimen in dilute spirit, a groove would result, and this is probably what 

 was seen and described by Goodsir as the "primitive dental groove": 

 but, as the student will gather from the text, there is at no time any such 

 thing as a deep open groove like that described by him, unless it results, 

 from maceration and consequent partial destruction of the specimen. 



K 



