THE TEETH 



329 



rapid succession, as many as ten being visible at one time in the 

 ordinary Viper. As the tooth-germs develop and approach the 

 surface they take up a position in two rows (3, 5, and 2, 4, in Fig. 

 163). 



The maxilla, which carries the functional fang, has two bases of 

 attachment for teeth, an inner and an outer, and these are made use 

 of alternately — a functional 

 tooth with external attach- 

 ment being succeeded by 

 one with internal and con- 

 versely. 



The replacement takes 

 place approximately syn- 

 chronously in the two 

 maxillae a pair of teeth 

 attached to the right-hand 

 bases of attachment of the 

 two maxillae (in other 

 words attached to the outer 

 base of attachment on the 

 right maxilla and to the 

 inner base of attachment 

 on the left maxilla) being 

 replaced by a pair attached 

 to the left-hand bases of 

 attachment (inner on right 

 maxilla, outer on left max- 

 illa). In consequence of this 

 arrangement the individual 

 teeth of a functional pair 

 are the same distance from 

 one another as their prede- 



arrangement of the replace- 

 ment teeth we have doubt- 



Fig. 163. — Part of transverse section through upper 

 jaw of a young Viper. (After Rose, 1894.) 



The developing teeth are numbered in order of sequence. 



No. 8 is not shown. Dark tone = ectoderm ; pale tone = 



mesenchyme. Dentine is shown in black. The enamel 



which is present as a thin sheath over the apical end of the 



CeSSOrS and their SUCCeSSOrS tooth is not shown. In 7 the poison canal is seen as an open 



t ,i ■ -i-n p groove along the side of the tooth, filled with ectoderm ; in 



in this rnOQincatlOn Of G this has become converted into a tube (jj.c) still filled with 



the primitive linear ectoderm cells; in 2 the cells have degenerated, leaving a 



clear lumen ; in 1 all of the ectoderm external to the tooth 

 except its outermost layer has broken dpwn to form the 

 cavity of the tooth-sheath (S). The functional tooth (1) in 

 leSS a mechanism tO Secure ^ ne section here figured belongs to the outer series, to which 



more ranid qupppssion and 3 and 5 also belong ' dA ' dental lamina ; p ' p,Up ; px ' poison 

 more rapid succession, ana canal . g _ tooth . sheath . „_ bl00d . vesse i. 



in this connexion it is of 



interest to notice that the replacement of the functional tooth is 

 not dependent upon its having already suffered injury or become 

 worn out but takes place at regular intervals (about six weeks 

 in the case of the European Viper, Kathariner) while the snake 

 is leading an active life. 



In the Crocodiles the dental lamina becomes broken up into a 

 network, and finally reduced to a strand of tissue running longitudin- 

 ally along the jaw, slightly to the inner side of the tooth-bases. A 



