298 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



of a continuous series (Fig. 123) : the fact receives further demonstration 

 from the study of development which shows that early stages of the 

 two organs are identical in appearance. 



A remarkable arrangement is present whereby the teeth as they 

 become worn down or broken off are replaced by new teeth. The tooth- 

 bearing strip of skin covering the jaw is bounded all along its outer 

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



It is bounded similarly along its 

 .t inner margin by a formative 



zonCj 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- 

 tions of the fins, both median 

 and paired, are supported by 

 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 

 therefore to a certain extent cognate with the placoid scales. 



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

 terized in the Dogfish, as in all other Vertebrates during at least a portion 

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

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

 cleft being vertical in position and passing outwards through the entire 



Fig. 123. 



Transverse sectjon through lower jaw of an em- 

 bryo Dogfish (ScyUutm) showing the gradual transition 

 between placoid scales and teeth., {The Cambridge 

 Natural HUtory, vol. vii. ; from Gegenbaur.) c, Carti- 

 lage of lower jaw ; d, placoid scales ; t, teeth. 



