432 PRACTICAL DIBBCTIONS. [APP. 



Well hardened tissues may be left in this 

 aqueous solution for 24 hours. It is especially 

 good for staining nuclei ; as a rule the staining 

 is not diffuse, but it is necessary after staining 

 to treat with acid alcohol (see a). 



3. Imbedding and Cutting Sections. 



It is not possible to obtain satisfactory sec- 

 tions of embryos without employing some 

 method of imbedding, and using a microtome. 

 Many imbedding solutions and methods of cut- 

 ting sections have been used, but we find the 

 following far superior to any other. It combines 

 several advantages ; in the first place it renders 

 it comparatively easy to obtain, what is so 

 essential, a complete consecutive series of sec- 

 tions of the embryo ; and secondly, all the sec- 

 tions when mounted are in the same relative 

 position ; and the various parts of each section 

 retain their normal position with regard to 

 each other. 



a. Imbedding. 



The substance we prefer for imbedding is 

 paraflSn. As will be seen below it is necessary 

 to have at hand paraffins of various melting 

 points, according to the temperature of the 

 room at the time when the sections are cut. 



It will be found most convenient to obtain 

 paraffins of the highest and lowest melting 

 points and to mix them together as experience 

 dictates. 



Place the stained embryo in absolute alco- 

 hol until completely dehydrated (two hours is 

 sufficient for small embryos) : and when ready 



