LATERAL CANAL SYSTEM OF SELACHIANS 5 



the bases of the 'hairs.' Although subsequent researches with 

 the help of better histological methods have shown that Schulze 

 was wrong in reference to the termination of the nerve fibers, 

 still, we have foreshadowed in this paper the most important 

 question regarding the sense organs — the existence of club- 

 shaped sensory cells with hair-like processes and the relation 

 or nerve fibers to these sensory cells. 



Then followed, in 1868, Leydig's paper on organs of a sixth 

 sense, in which he extended his observations to skin organs of 

 amphibians and reptiles. He gives a comprehensive review of 

 the early literature, including his own work of 1850 on the canal 

 organs of teleost fishes, but in this paper he does not give 

 sketches of the structural condition in fishes. The figures are 

 limited to amphibia and certain skin organs of reptiles. 



Schulze's paper of 1870 is fully illustrated for young teleosts 

 and amphibia and made more clear and definite that, structural- 

 ly, the chief characteristic of these sense organs is the presence 

 of pear-shaped hair-cells, and that the nature of nerve termina- 

 tions is the fundamental point of investigation. 



By these researches the ground had been cleared, and the 

 question outlined for nem-ological investigation was the histo- 

 logical structure of the sense organs and the nature of nerve 

 terminations. But investigations along these lines were re- 

 tarded for lack of proper histological technique and reliable 

 methods of impregnation of nerve fibers. 



All this, also, was before the demonstrations of His upon the 

 origin of nerve fibers ('88) and the formulation of the neuron 

 theory by Waldeyer ('91). Regarding the nerve terminations 

 there arose a period of discussion as to whether there was con- 

 tinuity (Merkel '80) or merely contiguity between nerve fibers 

 and sensory cells. 



On account of the close generic resemblance between the sense 

 organs of the lateral line and those of the ear it will not be out 

 of place to mention in this connection two papers on termina- 

 tions of the eighth nerve. Among others, Ayers ('90) represents 

 the conception of continuity (his diagram, Plate XH) between 

 ganglionic fibers and sensory hair-cells of the internal ear. 



