DERMAL SKELETON. 



31 



appear much earlier in the developing animal than do definite 

 ossifications within particular parts of the cartilaginous skeleton. 

 The condition which obtains in Fishes and Amphibia well illustrates 



FIG. 18. DERM. 



;CLE> OF Protoptcrus. 



D, the apical portion of the denticle ; S, S, the base of the denticle, the cavity of 

 which is seen in optical section through the transparent apical portion at S.I and S.2. 



this. To take a single example : the young File-fish (Balistes) is 

 provided with a complete dermal armour at the time when the 

 ossification of the primordial cranium has hardly begun. 



FIG. 19. DERMAL ARMATURE OF Callichtliys. 



B, barbules ; BrF, pectoral fin ; BF, pelvic fin ; RF, dorsal fin ; DS and FS, dorsal 

 and ventral bony shields ; f, lateral line. 



The exoskeleton originates by the formation of small denticles 

 (Figs. 17 and 18) attached to basal plates, which lie scattered over 



