4 METHODS EMPLOYED. 



XXXIII., XXXVI., XLV., XLVI., XLVIII., LIII. & LVIIL, 

 which were taken from specimens hardened first in chromic acid, the 

 preparations used for the illustrations have all been obtained by the same 

 process. 



Both the brain and spinal cord were entirely separated from the 

 body, and, with their membranes, placed in iodine tinted alcohol until 

 they had acquired a slight degree of consistency from six to twelve 

 hours. They were then transferred to a 3-100 solution of bichrom- 

 ate of potash, with a small piece of camphor, in a tightly corked, wide- 

 mouthed bottle, and allowed to remain until ready for cutting, renewing 

 the solution every two weeks. 



The time required for the hardening process varies considerably in 

 different animals, and this variation is more dependent upon the class of 

 the animal than upon the relative dimensions of the specimens. 



For example : on the same day, I placed the brain of a large rattle- 

 snake with that of a small salamander in the same bottle, and at the 

 end of six weeks, the former was ready for section, while the latter was 

 not sufficiently hard until a month afterwards. By thus employing the 

 same reagents in all cases, I have been able to note constant differen- 

 ces in the action of both the hardening and the coloring agent, carmine. 



Perhaps the most striking illustration of this is furnished by the 

 nervous centres of tailed batrachians, which, while they stain very read- 

 ily, invariably require about a third more time to harden than spec- 

 imens from the other orders. Specimens from ophidians stain less sat- 

 isfactorily than those from any other of the classes which I have studied, 

 while with the spinal cords of alligators, turtles and frogs, failure to obtain 

 good results, in this particular, is very rare. 



In all cases the sections have been stained after cutting, injury from 

 excessive handling being wholly avoided by the use of siphon tubes to 



