426 PRACTICAL DIRECTIONS. [APP. 



should be placed in 90 p.c. alcohol and kept 

 there until required for further use. 



N.B. Hardened embryos should always be 

 kept in 90 p.c. spirit and only placed in abso- 

 lute before imbedding, or staining with liaema- 

 toxylin. 



Some liistologists prefer to keep hardened tissues 

 in alcohol 70 p.c. 



b. Corrosive sublimate. 



Place the embryo in a large quantity of a 

 saturated aqueous solution of corrosive subli- 

 mate to which a few drops of glacial acetic acid 

 have been added, and allow it to remain for 

 half-an-hour'. It is necessary thoroughly to ex- 

 tract the corrosive sublimate from the cells of the 

 embryo ; to accomplish this, wash it thoroughly 

 with water for from 10 minutes to 3 hours ac- 

 cording to the size of the object. The washing 

 may be limited to frequent changes of water or 

 the embryo may be placed in a vessel through 

 which a continuous stream of water is kept 

 running. When all the sublimate is removed, 

 place it in 50 p.c. alcohol acidulated with nitric 

 acid (half-a-dozen drops of acid to a 4 oz. 

 bottle of spirit) for five minutes. The preser- 

 vation of the embryo is completed by treating 

 it with 70 p.c. alcohol for twenty-four hours and 

 then kee[)ing it in 90 p.c. alcohol. We have 

 not found that corrosive sublimate gives such 

 good results as picric acid in the case of chicks 

 and mammalian embryos. 



1 If there is only a small quantity of acetic acid mixed with the 

 sublimate, a prolonged immersion will do the embryo no harm. 



