TIIK TKKTII 



329 



r;ij)id succession, ;is many as tm being visible at one time in tin- 

 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 faiijj, has two bases of 

 attachment for teeth, an inner and an outer, and lhcs<- arc made use 

 of alternately a functional 



9 f 7- dl 



p.c 



6, 



ir 



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



cessors and their successors. tooth is not shmvn \ In . 7 , the 1>oison ;,^'! al is . seen ar \ an open 



In this modification of 

 the primitive 



P 



dl 



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 = 

 iiiesem-hvme. Dmtine is shown in black. The enamel 



arrangement of the replace- 



groove along the side of the tooth, filled with ectoderm ; in 

 C this has become converted into a tube (p.c) still tilled with 

 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 lias broken down to form the 



teeth We have dOUbt- cavity of the tooth-sheath (S). The functional tooth (1) in 

 a mechanism tO SeCUre thesection here liguml belongs to the outer series t., which 



more rapid succession, and 



in this connexion it is ol 



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 



