240 PRACTICAL DIRECTIONS. [APP. 



the embryo, both as a transparent, and as an opaque 

 object, seen from below as well as from above. 

 These sketches will be of great service to him when 

 he comes to study the sections of the same embryo. 



G. On prcpct7'ing sections of the embryo. 



1. Hardening. 



a. With chromic acid. 



The embryo must be immersed in the way 

 recommended under F, in a solution of the 

 strength of "1 p. c. for 24 hours. From this it 

 should be removed and placed in a stronger 

 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 transferred to 

 alcohol of 90 p. c, after remaining in which for 

 two days, it should lastly be placed in absolute 

 alcohol ; in this it can stay till required for section. 

 If the chromic acid has not by this time been com- 

 pletely got rid of, the absolute alcohol must be 

 changed, till the specimen is entirely free from 

 acid. 



b. With picric acid. 



The best form for applying the picric acid, is 

 that introduced by Dr Kleinenberg. 



With 100 parts of Avater, make a cold saturated 

 solution of picric acid ; add to this two parts of 

 concentrated sulphuric acid : filter and add to the 

 filtrate three times its bulk of water. 



In this solution of picric acid, the embryo 

 must be immersed in the same manner as when 

 chromic acid is used (vide F). After remain- 

 ing in the acid for Jive hours, it is to be treated 

 successively with weak, strong and absolute alco- 

 hol, as was done in the case of the chromic acid. 

 Still more difficulty will probably be found in 

 getting rid of the picric acid, than was found to 



