34 



THE ELASMOBRANCH FISHES 



Marker! (1896) says that in the formation of this secondary dentine certain 

 long fibers {fs., fig. 37a) grow down in a sheath posterior to the core of carti- 

 lage (ct.). These fibers penetrate deeply and near the base spread out, curve 

 forward, and fuse anteriorly into a closed ring. Now this posterior sheath of 

 fibers runs through the odontoblasts in such a way as to leave some of them in 





^s 







A. Young stage 



B. Older stage 

 Fig. 37. Development of fin spine, Acanthias. (From Markert.) 



ct., core of cartilage; d., primary dentine; d.', secondary dentine; e., enamel; e.o., 

 enamel organ; ep., epidermis; fs., connective tissue fibers; od., odontoblast; pg., pigment. 



front of it and others behind it. Both of these sets of odontoblasts lay down 

 dentine so that the fibers (fs.) come to lie between two layers of dentine. 

 Finally the fibers themselves also give place to dentine. Hence a transverse 

 section of this secondary dentine, taken at the base of the spine where the 

 sheath of fibers is closed into a ring, shows a heavy circle of secondary dentine 

 like the broad band around the core of cartilage {d\ fig. 40) ; a similar trans- 

 verse section taken toward the tip would indicate the layer of dentine as a 

 crescent just back of the central core. 



