328 EMBBYOLOGY OF THE LOWEE VEETEBEATES ch. 



marked by a slight indentation of the surface in Fig. 162, B), while 

 there takes place a compensating process of growth, or formation of 

 new skin, along the inner margin of the jaw, near the bottom of a 

 deep groove (Fig. 162, A, d.g). The young skin arising in this 

 region, like young skin elsewhere, produces placoid elements and it 

 is these which, growing older as they gradually move forward over 

 the jaw surface, become the functional teeth. Under normal pircum- 

 stances the rate of outward progress is such that by the time the 

 tooth is becoming inefficient through wear it gets into the absorptive 

 .region and is shed. 



It should be mentioned that, to make it more easily intelligible, 

 the above account has been simplified in one important detail. The 

 replacement groove is as a matter of fact in the Elasmobranchs 

 mentioned no longer an open groove. Its walls have become fused 

 together, so as to obliterate its cavity and form a solid lamina of 

 ectoderm which dips down into the mesenchyme round the boundary 

 of the mouth, just within the jaw, as shown in Fig. 162, B. 



In Amphibia the general arrangements for replacement of the 

 teeth are similar to those of Elasmobranchs, as may be gathered from 

 Fig. 162, C, but an advance beyond the Elasmobranch condition is 

 found here in that the functional tooth is firmly fused to the jaw. 

 It remains stationary throughout its period of active functioning 

 and it is only at the end of that period, when it is lost either by 

 being broken off or by a process of natural shedding accompanied by 

 resorption, that the next replacement tooth in the series moves up 

 to take its place. 



In Eeptiles (Fig. 162, D) again the general arrangement is 

 similar except that economizing of material has now taken place, 

 the dental lamina being relatively reduced in bulk between the tooth- 

 germs, so that the latter project prominently from the lamina 

 instead of being embedded in its substance as was the case in the 

 Amphibian. 



Amongst the Lizards certain modifications occur which are of 

 importance as foreshadowing arrangements which occur in Mammals. 

 Thus it may happen (Iguana, Leche, 1893) that the first generation 

 of teeth to be formed never become functional but disappear before, 

 hatching. Again the replacement mechanism may become reduced in 

 the anterior part of the series (Agama — Carlsson, 1896 ; Chamaeleo — 

 Eose, 1893). In the Chameleon the ordinary replacement mechanism 

 is no longer functional except at the extreme hind end of the jaw, 

 where alone new teeth are produced. 



The large poison-fangs of poisonous snakes are peculiarly liable 

 to injury and we find, as might be expected, that the replacement 

 mechanism is in their case particularly well developed. The dental 

 lamina (Fig. 163, d.T), which is very extensive and thinned down to 

 such an extent as to become perforated by numerous openings in its 

 more superficial and no longer active portions, is curved scrollwise 

 upon itself and upon its concave surface develops tooth-germs in 



