12 SYDNEY EVANS JOHNSON 



This treatment was found to be of great advantage as it 

 facilitates sectioning the tough dog-fish integument and also 

 prevents deposits of silver in the connective tissue elements, a 

 drawback which was encountered in the use of other silver re- 

 duction methods. 



The pieces of tissue are again rinsed in distilled water and 

 then dehydrated in 80 per cent, 95 per cent, and absolute am- 

 moniacal alcohol. From the absolute alcohol the pieces are 

 put directly into pyridine. In this they remain for twenty-four 

 hours, and are then washed for the same length of time in run- 

 ning water. The tissues are then washed for half an hour in 

 distilled water and put into 2 per cent silver nitrate solution, 

 at 37°C., for two to seven days. When the pieces of tissue hi 

 this solution have acquired the proper color they are rinsed 

 briefly in distilled water, reduced in a 2 per cent solution of 

 hydrochinon in 4 per cent formol, dehydrated rapidly, cleared 

 in benzol, and unbedded in paraffin. 



After treatment with the nitric acid solution it was often 

 found advantageous to remo^'e the sensory canal from its 'der- 

 mal tunnel.' This could be done quite readily with the aid of 

 a pair of sharp needles and a dissecting microscope. 



The greatest disadvantage of the method as outlined above 

 is due to shrinkage of tissue and the failure to show cell bound- 

 aries in the sensory epithelium. The cell boundaries, however, 



Fig. -1 Diagram illustrating the general relations of the lateral nerve and its 

 ramuli to the lateral sensory canal with its surface tubules. Drawn from cam- 

 era sketches except that the distance between serial ramuli and tubuli lias been 

 greatly foreshortened. 



Fig. 5 Longitudinal section of the lateral canal of a Mustelus 'pup' at the 

 level of the first dorsal fin. This semidiagrammatic figure is a composite, sev- 

 eral preparations having been utilized in its construction. The nerve fibers 

 and fibrillae were drawn from pyridine silver and the cellular parts from haema- 

 toxj'lin preparations. The sketch shows the nerve distribution and the general 

 histological structure of the sensory epithelium. X 240. 



Fig. 6 Transverse section of lateral canal at the site of a suiface tubule. 

 The tubule passes through the dermis in a ventro-lateral direction, opening on 

 the surface near the base of the slightly modified scale. Mustelus canis pup. 

 X 85. 



Fig. 7 From the same specimen as tlic alcove, more highly magnified. X 260. 



