APP.] HARDENING EMBRYOS. 427 



c. Osmic acid. 



Osmic acid is a difficult reagent to use, but 

 when properly applied it gives most excellent 

 results. 



It should be used as a weak solution (! to 

 5 p.c). The object should be left in it until 

 it has acquired a light brown tint. The stronger 

 the solution the less time is required for the 

 production of this tint. It should then be 

 removed and placed in picro- carmine, which 

 arrests the action of the osmic and stains the 

 embryo. The time required for the picro-car- 

 mine staining must be determined by practice. 

 From the picro-carmine the object must be 

 washed in 70 p.c. spirit; and then placed in 

 90, or may be preserved directly in glycerine. 



If it is desired to use other staining agents 

 (borax-carmine is good for some preparations), 

 the object must be removed from osmic into 

 water or weak spirit, thence through 50 into 

 70 p.c, stained, and passed through 70 to 

 90 p.c. spirit. 



d. After using osmic it is well in some cases 

 (mammalian segmenting ova) to place the 

 object in Miiller's fluid for 2 or 3 days, after 

 which it may be preserved in glycerine or spirit. 



Miiller's fluid is made by dissolving 25 grms. 

 of bichromate of potash and 10 grms. of sodic 

 sulphate in 1000 cc. of water. 



e. With chromic acid. 



The embryo must be immersed in a solution 

 of the strength of ! p.c. for 24 hours. From 

 this it should be removed and placed in a stronger 



