ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



Jb 



. ontinuous series (Fig. 123) : the fact receives further demonstration 

 I n.in the study of development which shows that early stages of the 



identical in appearance. 



A remarkable arrangement is present whereby the teeth as they 

 me \v.rn down or broken off are replaced by new teeth. The tooth- 

 trip of skin covering the jaw is bounded all along its outer 

 edge by a lint- along which gradual absorption of the skin is taking place. 



It is bounded similarly along its 

 inner margin by a formative 

 zone, dipping deep down into 

 the lining of the mouth,, along 

 which active growth of the skin 

 is taking place. The result of 

 this arrangement is that the 

 tooth-bearing strip of skin is 

 throughout the life of the 

 creature undergoing a gradual 

 outward movement,, sliding as 

 it were with extreme slowness 

 over the jaw surface. The 

 rate of this movement is nicely 

 adapted to the normal rate of 

 wear and tear of the teeth. A 

 new tooth arising in the for- 

 mative zone travels slowly out- 

 wards and after about the 

 average period of usefulness it 

 reaches the absorptive zone and 

 is shed. 



The thin marginal por- 



d- 





lower jaw of an em- 



-I>'//I'K)) showing the ^nulual transition 



teeth. (The Cambridge tions of the fins, both median 



Natural //is/<>rv. vol. vii. ; from Gi-j-ciibuur.) c, Carti- j ^^p^ Qrp cnnnnrted bv 



(h and paired, are suppoi 



numerous tough rays or fila- 



ments of horny appearance. These originate in the embryo in close 

 relation with the basement membrane underlying the ectoderm and are 



nain extent cognate with the placoid scales. 



The plmrynx, into which the buccal cavity is continued, is charac- 



tcri/eo! in the I )o-fisl. as in all other Vertebrates during at least a portion 



of their developmental history, by the presence of the gill-clefts or 



branchial .lefts. Of ordinary branchial clefts there are five pairs, each 



'rtical i n position and passing outwards through the entire 



