16 

 upon Viustelus, it /las soon found that strict account liad to be 

 taken of the physiolofe'i c&l state of the animal. An individual 

 fresh from the pursuit of his prey in the open sea ^ave alto- 

 gether different results from one which had been kept for some 

 time in a small aquarium, simply because the reactions of the 

 tissues in the two instances were quite different. And so, 

 before even fair results with chrome-silver impregnation could 

 be secured, it was necessary to make a careful study of physio- 

 logical conditions. 



It was also found that selachian nervous elements lend 

 themselves but grudgingly, at best, to the reaction desired. 

 Numerous trials were made of the several published schemes for 

 securing impregnation, but particularly of the procedure indi- 

 cated by Golgi ('94); by Ramon y Cajal ('94); by Flechsig ('89); 

 by Cox ('91); and by Strong {'9^, '96). 5.very application 

 practicable was also made of formaldehyde as a constituent of 

 the reagents employed. A general critique of these processes 

 has already been given by me in a former paper ('97a). 



The slices of perfectly fresn brain from the most active 

 animal procuratie were placed in the "rapid" hardening mixture 

 of Golgi. The pieces were always small, not over two millimeters 

 in thickness for, e.g., a transverse section of the forebrain. 

 The proportion of the hardening fluid used embraced one part 

 of 1% osmic acid to four parts c.5% potassium bichromate, and 

 this reagent was used in liberal quantities. The proper dura- 

 tion of hardening was influenced by the temperature of the 



