326 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



Ganoine to distinguish it from the true enamel of the higher Verte- 

 brates. 



As regards the basal plate the main question at issue is the 

 evolutionary one whether or not the view of Gegenbaur should be 

 accepted that these basal plates constitute the first phase in the 

 evolution of the bony skeleton. This question will more suitably be 

 discussed in connexion with the bony skeleton in general. 



Lastly questions of general interest are raised by these excep- 

 tional cases where the developing tooth cannot be traced into 

 immediate relationship with the ectoderm. In Lepidosiren and 

 Protopterus as well as in the Urodele Amphibians portions of the 

 lining of the buccal cavity which give rise to the teeth have the 

 appearance of being derived in the embryo from endoderm. Again 

 in Teleostean fishes teeth are developed far back in the pharyngeal 

 region, in other words in a portion of the alimentary canal which is 

 lined with endoderm. 



Such cases obviously cause serious trouble to those who apply 

 the germ layer theory rigidly. They explain them by supposing 

 that there takes place in development an actual spreading inwards 

 of ectoderm over the surfaces on which teeth will develop. As 

 indicated in Chapter III. in dealing with the buccal lining of 

 Urodeles and Lung-fishes the writer of this volume believes that the 

 evidence adduced so far of the ingrowth required by this explanation 

 is not to be relied upon. He would rather explain such cases as 

 due to the more or less broad debatable zone between the ectoderm 

 and endoderm, the influence of one layer being liable to spread into 

 the other and there being no sharp line the position in regard to 

 which decides definitely to which layer a particular organ belongs. 



Egg-Tooth of Reptilia. — In the embryos of Eep tiles there 

 appears a precociously developed "egg -tooth" at the tip of the 

 upper jaw which has for its function the rupture of the egg-shell. 

 In Geckos there are a pair of these present, attached to the pre- 

 maxilla close to the mesial plane. In other Eeptiles the left egg- 

 tooth appears only as a transient rudiment and the functional (right) 

 tooth takes up a practically median position so that it appears to be 

 unpaired. It is of interest that this holds also for snakes in which 

 there are no definitive teeth in the premaxillary region (Rose, 1894). 



Poison Fang of Vipepjdae. — The poison fang of the Viperidae 

 is highly specialized for the injection of poison, its pulp being 

 traversed by a longitudinal tube, composed of dentine and attache'd 

 to the outer wall of the tooth along its anterior face. The inner 

 tube — or poison canal — passes at each end into an open groove the 

 openings so formed serving for entrance and exit of the poison 

 respectively. The main features , of the development are illustrated 

 by the transverse sections shown in Fig. 163 (p. 329). The poison 

 canal makes its appearance as a longitudinal infolding of the dentine, 

 the ectoderm of the tooth-germ seeming to push the dentine in 

 before it to form a groove (Fig. 163, 7). The groove deepens and its 



