426 J. (ilfAJfAM Kl']\{li. 



enamel organ. It is difHcnlt to arrive at a certain opinion on 

 the morpliological nature of this first formed cap. It adheres 

 strongly to the enamel organ, as shown by the torn surfaces 

 when the two structuies have been pulled apart in process of 

 preparation, and in many cases it is for a time sharply marked 

 off from the underlying dentine. On the other hand, it 

 differs fron) ordinary enamel in the much larger proportion 

 of organic matter, which causes it to remain quite distinct 

 even in decalcified specimens. 



On the whole, I am inclined to look upon this structure as 

 being enamel, though of a somewhat modified kind. 



The structure of the palatopterygoid teeth about Stage 35 

 maybe gathered from the sections represented in figs. 7 b, 7 c, 

 and 7 d. The enamel forms a distinct cap tapering off 

 towards its edges, and sharply marked off from the under- 

 lying dentine. It shows a faint striation perpendicular to its 

 surface. In undistorted sections the flat inner ends of the 

 enamel cells abut close against it (fig. 7 d). 



At this stage there is a quite definite though still thin 

 layer of dentine lying within the enamel cap. The broadened 

 outer ends of the odontoblasts come into close contact with 

 one another, and form, to the eye, a quite continuous mass 

 (fig. 7 d). As they pass into tins their protoplasm shows a 

 development of fine fibrillas crossing one another in all 

 directions. Traced still further out the fibrillar mass gradu- 

 ally takes on more and more deeply the stain which, in 

 Heidenhain's htumatoxylin preparations, indicates the pre- 

 sence of calcareous matter. The thin outer layer is, in fact, 

 fully calcified dfuiine, on its iuncM- si(h' ])iissing by impercep- 

 tible gradations into tlii' oi'dinaiy ])]'otoplasm of the odonto- 

 blast.s, on its outer side niarked off from the enamel by a 

 sharp boundary. 



In Stage 36 (fig. 7 e) the formation both of dentine and of 

 the bony trabeculu' which form the spongy basal support for 

 the tooth is seen lo have nuidc considerable progress. Tlic 

 ridges of the tooth now approach the surface of the oral 

 epithelium, which is becoming lliin over their ;i])ic(\s -[irejinra- 



