322 EMBRYOLOGY OF THE LOWEE VERTEBRATES CH. 



the epidermis. Presently this begins to bulge upwards like a dome 

 into the epidermis. The epidermal cells immediately bounding this 

 little dermal elevation take on a columnar shape : they constitute 

 the enamel epithelium (Fig. 160, e). The (dermal) cells on the 

 surface of the dome or papilla, immediately underlying the enamel 

 epithelium, also become distinct and form a definite layer of odonto- 

 blasts. 



The hard substance of the scale makes its appearance as a cone 

 of dentine fitting over the surface of the dermal papilla and in turn 

 ensheathed by the enamel epithelium. The dentine cone, which 

 usually becomes directed tailwards, gradually thickens, encroaching 

 upon the dermal papilla or pulp which it surrounds. It lies 



FIG. 160. Longitudinal vertical section through the skin of an embryonic Shark to show 

 a developing placoid scale. (From Balfour's Emlniology \ figure by Gegeubaur a tin 

 Hertwig, 1874.) 



/ . .-]!. I.TIIIIS : . <-namel epithelium; o, enamel; /<. di-imal papilla. 



immediately outside the odontoblasts and as it increases in thickness 

 the outer portion of some of the odontoblasts persists as a fine 

 thread of cytoplasm extending out through the substance of the 

 dentine, so that when dried the dentine is seen to be traversed by 

 innumerable fine slightly diverging canals each of which contained a 

 protoplasmic thread. 



Tin: hard nuiterial of the dentine is commonly regarded as 

 calcifi<-<! matrix but there is evidence which points rather to its 

 hein;_r formed of modified cell cytoplasm. 'Phis point will be returned 

 10 in eonnexjon with tin- development of the teeth. 



Towardfl the Surface of llic cone the calcified siihstanc.e changes 



its character. It becomes extremely hard (consisting of very dense 



;m carhonate), transparent and highly refract in-, ami the 



terminal branch,- of the tuhules \\ithin it arc reduced to an extreme 



degree of fineness. This outer Ia\cr i^ commonly known as enamel. 



It may he comparat i \ ! y thick, or on the other hand it may he 

 :ne|\ thin a.s for example in t he scales of Af<t //////'"-> except the 



