428 PRACTICAL DIRECTIONS. [APP. 



solution (-3 p.c.) for another 24 hours. If it 

 then appears sufficiently hard, it may be at 

 once placed in alcohol of 70 p.c., in which it 

 should remain for one day, and then be trans- 

 ferred to alcohol of 90 p.c. 



f. Absolute alcohol has also been employed as 



a hardening reagent, but is by no means so good 

 as the reagents recommended above. 



The object of these so-called hardening reagents is 

 to kill the tissues with the greatest possible rapidity 

 without thereby destroying them. The subsequent 

 treatment with alcohol completes the hardening which 

 is only commenced by these reagents. 



There is room for the exercise of considerable skill 

 in the use of alcohol, and this skill can only be acquired 

 by experience. A few general rules may however be 

 laid down. 



(1) Tissues should not, generally, be changed from water 

 or an aqueous solution of the first hardening reagent 

 into an alcoholic solution of too great strength, nor 

 should the successive solutions of alcohol used differ 

 too much in strength. The distortion produced by 

 the violence and inequality of the diffusion currents 

 is thus diminished. This general rule should be 

 remembered in transferring tissues from alcohol to 

 the staining agents and vice versa. 



(2) The tissues should not be left too long (more than 

 one or two hours) in alcoholic solutions containing 

 less than 70 p.c. of alcohol. 



(3) They should not be kept in absolute alcohol longer 

 than is necessary to dehydrate them (see B. 1, p. 426). 

 The alcoholic solutions we generally use contain 30, 

 50, 70, 90 p.c. of alcohol. 



2. STAINING. 



In most cases it will be found of advantage 

 to stain the embryo. The best method of doing 



