10 



ARKIV FÖR ZOOLOGI. BAND 7. NIO 2. 



layer of cement, on tlie other hand, correspond with eacli 

 otlier and represent a somewhat låter stage of development, 

 i. e. wlien the permanent growth of the tooth already has 

 begun. 



An examination of a section 

 through a somewhat older and worn ^ 

 tooth (from the skull with 47 Vs cm. 

 basal length) proves that the deve- 

 lopment continues as it has begun 

 (fig. 4). The greater part of the 

 crown, which projects beyond the 

 tube of cement, is worn away, and 

 even the cement itself is partly worn 

 off in such a way that is flush with 

 the remains of the crown, and the 

 tooth t hus forms a bluntly rounded 

 cone. It is, however, the dentine 

 of the basal portion of the crown, 

 which still forms the apex of the 

 tooth, and on the section this layer 

 of dentine is recognizable on its 

 comparative homogeneousness. Be- 

 neath this follows layer upon layer 

 of dentine of varying thickness, and 

 more or less defined from each other. 

 These layers are parallel with the 

 ijjpf |M^-/ basal surface of the first apical layer 



jjm W/ of dentine described above from the 



w Wf .young tooth, thus v/ith their apical 



J ]/ end in the centre and from there 



sloping outwards and downwards to 

 the outer surrounding cylinder of 

 cement. By these layers of dentine 

 the original pulpcavity is gradually 

 more and more filled up- and trans- 

 formed to a solid cylinder of dentine 

 which constitutes the centre of the låter on functioning tooth 

 but at the same time the pulpcavity is prolonged at its base 



^ One of the most worn teeth of this skull has been chosen. 



- A narrow central fissure appears, however, to persist a verv long 

 time since the other parts have beeome solid. (Conf. what is said above 

 about a central fissure.) 



Fig. 4. Longitudinal sec- 

 tion through the somewhat 

 worn tooth of a young White 

 whale showing the strata of 

 dentine and cement, c cement, 

 d dentine. 



