LÖNNBERG; THE DEXTITION OF DELPHINAPTERUS LEUCAS. 9 



The apical of these, which seems to be the densest, forms with 

 its compact apical part the crown but extends also somewhat 

 down into the root where it is hollow and gradually tapers 

 in thickness and finally ceases at a distance from the apical 

 end of the cement-layer which is about equal to the distance 

 between the lat ter point and the tip of the crown. The space 

 within the walls formed by the basal portion of this layer 

 of dentine is occupied by another layer of dentine which 

 gives the impression of being less dense (conf. fig. 3). When 

 only littie magnified the line of demarkation between these 

 two principal layers of dentine is very visible but under a 

 more strongly magnifying lens the dentinal tubules can be 

 followed from one layer to the other. The difference between 

 the two layers cannot therefore be very important, and it is 

 probably only due to different periods of calcification. The 

 inner layer is again subdivided in several thinner layers, and 

 Avith higher magnifying power it can be seen on very thin 

 preparates that the whole of the dentine is composed of mi- 

 nute strata Avhich are parallel to the surface of the main 

 layers but form a right angle to the dentinal tubules. 



The outer and apical layer of dentine is the one first 

 formed, the inner is a secondary filling of the primary pulp- 

 cavity which progresses from the tip towards the base where 

 the layer, of course, is thinner. The cylinder of dentine has 

 a diameter of about 3 mm. at the middle of this tooth. 



The cylinder of cement shows two distinct principal 

 layers as well. The inner of these reaches from the base of 

 the crown or the apical end of the cylinder of cement and 

 tapers gradually from there in a basal direction till it reaches 

 on a level with the basal end of the first mentioned outer 

 or apical layer of dentine where it ceases at the same time 

 as the latter. The outer layer of cement again reaches as 

 far as the inner main layer of dentine, or to the basal rim 

 of the pulpcavity. At the apical end the cement somewhat 

 overlaps the enamel layer which, however, is very thin and 

 soon ceases altogether. 



From this description and the figure (fig. 3) it appears 

 clear that the apical, outer layer of dentine and the inner 

 layer of cement belong together and form a unit, which, no 

 doubt, represents the foetal tooth i. e. the one formed during 

 the foetal life. The inner layer of dentine and the outer 



