30 



THE ELASMOBRANCH FISHES 



decreasing in brilliancy. Oshima has also studied the luminosity, and has 

 noted that in Etmopterus the light was never produced spontaneously but was 

 emitted regularly upon the application of mechanical stimulation. Whether or 

 not the pigmented iris through contraction and expansion regulates the light 

 thus produced has not been sufficiently studied. 



Placoid Scales 



In figure 29, showing the layers of epidermis and corium, is also shown a devel- 

 oping placoid scale. The first indication of such a scale is the collection of a 

 group of cells in the upper layer of the corium to form a dermal papilla (d.i).). 



..I.e. 



Fig. 33. A. Section through a light organ or photophore, Spinax niger. (From Johann.) 

 B. Pigmentation of a photophore, Etmopterus. (From Oshima.) 

 i.s., blood sinus; gr., basal or germinative layer of epidermis; ir., iris; I.e., lens cell; 

 U.C., light cell. 



As the papilla grows upward it raises the basal layer of the epidermis, making 

 of it a cap, the enamel organ (e.o.). By continuous growth the cells of the 

 enamel organ assume the high cubical type with their nuclei located toward 

 the outer margin. 



The cells of the enamel organ form a layer of enamel over the tip of the 

 papilla; while the odontoblasts of the dermal papilla which lie most superfi- 

 ciall}^ are the first to lay down dentine. The first layer covers the tip and sides 

 of the papilla lying immediately under the thin layer of enamel. Then the 

 odontoblasts which are located deeper send out processes around which den- 

 tine is deposited. The canals thus formed for the processes themselves are the 

 beginning of the dentinal canals and into them the protoplasmic processes of 

 those odontoblasts lying still deeper will later enter as the formation of the 

 dentine continues. Thus it is that the dentine produced from without inward 

 becomes thickened, finally crowding the core of the papilla into narrow 

 compass. 



When a scale like the one described above reaches the surface the epidermal 

 layers are rubbed off from the tip, and the body of the scale then erupts. In a 

 more mature embryo than the one here described many such scales erupt at 

 about the same time and come to take up a definite arrangement in patterns in 



