488 STRUCTURE AND DEVELOPMENT OF TEETH, W. WALDEYER. 



the enamel organ afford an explanation of many of the peculiarities in 

 the course of the enamel fibres which have been mentioned above. 

 In the first place, the fine transverse striae which run in a circular 

 direction around the external surface of the enamel are directly refer- 

 rible to the papillae. For if, towards the end of the formation of the 

 enamel, the enamel pulp disappears, and the external and internal 

 epithelial cells again come into contact with each other, the papillary 

 processes make their mark on the enamel membrane, and naturally 

 also on the product of its calcification, the enamel. The transverse ele- 

 vations of the latter are thus of precisely the same nature as the well- 

 known fine striae of the nails. Moreover, since the greater part of 

 the enamel is formed before the enamel jelly has disappeared, and 

 therefore at a time when the enamel membrane already exhibits the 

 impressions of the papillae, we may reasonably refer many peculiarities 

 in the course of the enamel prisms, especially their decussations, 

 spiral course, and undulations, as well as their optical characteristics, 

 to the same cause. 



DENTINE AND CEMENT. As Dursy (67) maintained, the 

 first germ of the dentine appears in the dental sacculus as 

 a dark semi-lunar area at the bottom of the dental groove 

 that is to say, of the enamel germ coetaneously and continu- 

 ously with which it is developed along each half of the jaw 

 (see fig. 100). At certain points corresponding to the position 

 of the subsequent teeth the young structure develops in the 

 form of papillae projecting against the enamel germ, whilst the 

 remainder atrophies. The two horns of the semi-lunar mass 

 (seen in section) extend, from the base of the dental papilla, 

 some distance upwards, and embrace the dentine germ and the 

 enamel organ. This constitutes the first trace of the denial 

 sacculus, which at this period consists of tissue somewhat 

 richer in cells and vessels than the mucous tissue of the dental 

 groove. The dental sacculi are only well defined at the earlier 

 periods of the formation of the tooth. "When the process of 

 development is more advanced, it is impossible any longer to 

 distinguish a capsule-like layer of connective tissue around it. 

 Moreover the dentine germ is only a special division of the 

 mucous tissue of the dental groove, unusually rich in vessels 

 and cells. After it has attained a certain size, the olontoblasts 

 above described develop from the cells lying at the periphery, 



