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J. A. GRAY AND M. D. FRANCIS 



Fig. 22. An electron micrograph at low magnification showing an over-all 

 view of an undemineralized section of a lesion produced by a 24-hour exposure 

 at 4°C to a medium consisting of 0.10 m lactic acid and 0.03 m CaClo plus 6 

 per cent hydroxyethyl cellulose adjusted to pH 3.5, from the same sample as 

 Figs. 16 and 17. The enamel surface is in the upper right corner. The large 

 areas containing no enamel coincide with the subsurface decalcified region 

 seen in the microradiograph of Fig. 17. The relatively sound outer layer is still 

 heavily mineralized, but some channels through this region following the inter- 

 rod spaces are visible. The decalcification front approaching normal enamel is 

 on the left side and was very obviously moving along between the rods. Sections 

 of normal enamel have little or no space between the rods (upper left-hand 

 corner of this figure or lower right-hand corner of Fig. 36). (X 2000.) 



micrographs. The embedding material, methyl methacrylate, proved 

 suitable for this purpose. It was found that the methacrylate pene- 

 trated to the very depths of the lesion and filled extremely small 

 voids. The location of the methacrylate can be better visualized by 

 demineralizing the sections with hydrochloric acid. Not only were 

 extensive voids created in enamel during incipient carious lesion 

 fonnation, but these voids were readily available to diffusing ma- 

 terials. 



The lesions made in vitro advanced between the rods, along the 

 striae of Retzius, and across prism cross striations. By all present 



