HISTOLOGY OF ENAMEL ORGAN 397 



illaries branching and ramifying even among the distal ends 

 of the odontoblasts. It is the simplest matter to recognize 

 and trace the finest capillaries of the dental pulp for long dis- 

 tances as they course among the odontoblasts even at right angles 

 to the long axis of the cells. This being so, there should be at 

 least not much greater difficulty in recognizing similar cap- 

 illaries, if present, in the enamel pulp of the same sections. All 

 the evidence, critically considered, supports the conclusion that 

 blood vessels do not occur within the enamel organ of mammalian 

 teeth. 



SUMMARY 



1. The comparative histology of the enamel organs of the 

 incisor teeth of the white rat and of the kitten reveals decided 

 differences. The enamel organ of the rat's incisor is restricted 

 to the outer surface; it is relatively very thin, and becomes 

 modified by the impingement of capillary loops into a papillated 

 membrane. A stratum intermedium is not sharply differentiated. 

 In the case of the kitten, the enamel organ covers the entire 

 tooth, it is relatively thick, due to the presence of an extensive 

 stellate reticulum, it contains a well-differentiated stratum 

 intermedium, and papillae are limited to the extreme peripheral 

 portion. 



2. The restriction of the enamel organ to the outer surface of 

 the developing incisor tooth of the rat proves that this tissue 

 is not essential as a ^stimulative layer' for the production of 

 dentin. 



3. Neither in the case of the rat's incisor nor in that of the 

 kitten's teeth is the enamel organ vascularized. Capillaries do 

 not penetrate the stellate reticulum. They may indent the 

 surface of the enamel organ, thus producing a 'papillary layer.' 

 The tips of the capillaries may, especially in later stages, fuse at 

 a certain point with the cells of the outer enamel epithelium, 

 and so establish continuity between the capillary lumen and the 

 intercellular spaces of the outer tunic. Red blood corpuscles 

 may thus infiltrate the outer tunic. They are later ingested by 

 giant-cells formed by the fusion of cells of the outer enamel 

 epithelium. 



