2Q2 



HISTOLOGY 



groove. The anterior wall of this groove becomes a primitive dental 

 ridge. Over the surface of this ridge structures arise which are funda- 

 mentally similar to embry- 

 onic integumentary scales. 

 At the fundus of this dental 

 groove these structures are 

 continually forming, and 

 travel anteriorly by a move- 

 ment of the whole epithelium 

 towards the mouth of the 

 groove. In its earliest stage 

 the tooth appears as a pa- 



FIG. 255. The two earliest stages of tooth-formation ... - , . . 



in the fundus of the dental groove of a young dogfish, P"!* formed of a COre of 



Acanthias vulgaris. p., mesodermal papillae; ep., mesenchyme and an epider- 



basal layer of the stratified epithelium, x 80. i i .-, /T-,. \ r^-i 



mal sheath (Fig. 255). The 



columnar cells forming the lower stratum of the epidermis enlarge to 

 become the enamel-forming tissue. The other layers of the epidermis 

 disintegrate. In Figure 256 remains of epidermal (B, e.p.) cells are 

 lying over the enamel cells. The cells of this enamel tissue become 

 much taller; their nuclei move towards the distal ends of the cells, 

 and the cytoplasm becomes highly vacuolated at the proximal ends of 



FIG. 256. A, larger developing tooth of Acanthias than that shown in Figure 255. bl.v., blood 

 vessels beginning to enter the papilla. Longitudinal, vertical section X 80. B, enlarged 

 detail of A at x to show the enamel cells (en.c.) and the young layer of enamel (en.l.); e.p., 

 remains of epidermal cells; den. c., outer dentine cells of the papilla. No dentine is yet 

 deposited. X 400. 



the cells. The hardest part and outer layer of the crown, the enamel, 

 is elaborated by these cells and secreted at their proximal ends. These 



