v THE TEETH 327 



lips meet (6) so as to convert it into a tube. From its mode of 

 formation this tube is at first filled with ectoderm of the tooth- 

 germ. Eventually however this ectoderm disintegrates and leaves 

 an open tubular cavity (2). 



Up till this stage the tooth is still enclosed in the ectodermal 

 germ which has increased much in size (2) but eventually this 

 ectodermal mass also disintegrates with the exception of its outer- 

 most layer, so as to give rise to the cavity of the sheath in which 

 the functional tooth is contained. As the tooth becomes functional 

 this cavity comes to communicate with the duct of the poison-gland 

 so that it receives the poisonous secretion, and owing to the poison- 

 canal retaining the form of an open groove towards the basal end of 

 the tooth it in turn receives the poison from the cavity of the sheath. 



iks 



C. 



A, 



^ft)A^ 



Fig. 162. — Diagram illustrating tooth succession in an Elasmobranch (A, primitive, 

 B, existing condition) ; an Amphibian (C) ; anil a Reptile {Lacerla) (D). 



d.g, dental groove ; d.l, dental lamina ; p.s, placoid scale ; t, tooth. 



Enamel is present as a thin layer towards the point of the fang : 

 traced towards the base it passes into a simple fine cuticle-like basal 

 membrane. 



Succession of Teeth. — In cases where the teeth have become 

 restricted to special areas, and more particularly in cases where from 

 their shape and from the habits of their owner they are liable to be 

 broken off or damaged by wear or otherwise, it is usual to find a 

 special arrangement for the replacement of the lost or injured teeth 

 by new ones. Such an arrangement is seen in its simplest form in 

 an ordinary Dog-fish or Shark (Fig. 162, A and B). The portion of 

 skin — ectoderm with its strong and fibrous underlying dermis — to 

 which the tooth-bases are attached is gradually, by processes of 

 differential growth, caused to shift its position in an outward direc- 

 tion over the edge of the jaw which supports it. This is brought 

 about by the skin undergoing a continual slow process of absorption 

 or atrophy along the outer margin of the jaw (about the point 



