APP.] HARDENING AND STAINING. 247 



be the case with the chromic acid, and the 

 alcohol will probably have to be changed several 

 times. 



c. With osmic acid. 



Immerse the embryo in a '5 p. c. solution of 

 osmic acid ; leave it in this, covered and in the 

 dark, for 2J hours, and then place the embryo in 

 absolute alcohol, taking care completely to get 

 rid of the acid by washing several times with 

 alcohol. 



The osmic acid method has the advantage of 

 being simpler than the others, and also of leaving 

 the cells in a more natural condition. Its dis- 

 advantages are, that it is less certain, and further, 

 that it is necessary to cut sections of the embryo 

 the next day after it has been placed in the spirit. 

 Otherwise it becomes too brittle ; the sections are 

 then not so easy to make, and not so good. It has 

 the further disadvantage that the outlines of the 

 individual cells are not so clearly brought out as 

 with the other two reagents. 



Absolute alcohol has also been employed as a 

 hardening reagent, but is by no means so good 

 as the reagents recommended above. 



2. STAINING. 



In most cases it will be found of advantage to 

 stain the embryo. The best method of doing this, 

 is to stain the embryo as a whole, rather than to 

 stain the individual sections after they have been 

 cut. 



Either carmine or haematoxylin may be em- 

 ployed. For carmine, Beales' or some other alco- 

 holic solution is the best. Into this the embryo 

 may be removed directly from the absolute alcohol, 

 left in the carmine for 24 hours, and then placed 

 again in absolute alcohol for another 24 hours 

 before being cut. 



The best solution of hsematoxylin, one for which 



