386 H. E. JORDAN 



a thin stratum intermedium, and an outer tunic occur essen- 

 tially as described for the kitten's tooth. 



A third differential characteristic concerns the extensive 

 indentation or furrowing of the enamel organ of the rat's incisor. 

 The impinging capillaries push the outer enamel epithelium into 

 contact with the stratum intermedium, so that the resulting 

 'papillae' of the 'papillary layer' consist essentially of clusters 

 of cells of the outer tunic and the stratum intermedium (fig.. 6). 

 The cells of the outer tunic have taken on a more or less regular 

 fusiform shape, and the 'papillae' as a whole have an acidophilic 

 staining reaction. The papillary layer of the enamel organ of 

 the incisor of the rat accordingly involves the entire thickness 

 of the organ. It results from an inpushing of capillaries. Just 

 as in the case of the enamel organ of the cat, so in the rat the 

 endothelium of the capillaries at certain points may fuse inti- 

 mately with the cells of the outer tunic. This effects fenestration 

 of the capillary wall and a continuity of capillary lumen and inter- 

 cellular spaces, and as a result permits an infiltration of blood 

 corpuscles. Since the enamel organ as a whole is here very thin, 

 these extravasuclar ery throplastids penetrate as far as the stratum 

 intermedium. But no capillaries as such actually pierce the 

 enamel organ. Here also these intercellular red blood corpuscles 

 may be ingested by giant-cells formed from the cells of the enamel 

 germ. 



A clear understanding of the histologic relations of the enamel 

 organ to the developing incisor of the white rat demands a con- 

 sideration of the grosser features of these teeth. In the lower 

 jaw these two teeth grow almost horizontally in line with the 

 long axis of the jaw; they are only slightly curved. A transverse 

 section of the head and jaws will therefore cut these teeth approxi- 

 mately transverely. The two incisors of the upper jaw, on the 

 contrary, curve sharply along the lateral border of each nostril, 

 tracing in their growth a complete semicircle from base of 

 papilla to point of eruption. Thus at the root the direction of 

 the tooth is vertical, alongside the nostril it is almost horizontal, 

 and terminally it again turns almost into a vertical position. 

 The concavity of these curving teeth is of course on the lingual 



