EMBEDDING 67 



ding media. Those that fix homogeneously, without coagulation, 

 tend to render the internal parts of cells inaccessible to most 

 embedding media. Failure to enter cells by no means necessarily 

 renders an embedding medium useless, for it may enter inter- 

 cellular spaces and give considerable support during sectioning. 

 Three substances have been chosen to represent diff"erent sorts 

 of embedding media. These three are gelatine, paraffin wax, and 

 butyl methacrylate. They have been chosen partly because they 

 illustrate three very different processes of solidification, partly 

 because they are among the most valuable of all embedding 

 media. 



GELATINE 



The outstanding advantage of gelatine as an embedding medium 

 is that the tissues need not at any stage be dehydrated. It follows 

 that most lipids, even if unfixed, remain in the tissue and can be 

 coloured by lysochromes (p. 86). Distortion by dehydration is 

 also avoided. There are, however, several disadvantages in gela- 

 tine embedding, as ordinarily practised. The gelatine cannot be 

 removed from the tissue; and since it is capable of taking up both 

 basic and acid dyes, it often tends to obscure the view. The 

 ordinary process cannot be relied on to provide sections thinner 

 than about 5 //, and they do not adhere to form ribbons. It has 

 recently been claimed, however, that by modifying the procedure 

 one may obtain ribbons of sections only 20 m/n thick, suitable for 

 examination with the electron-microscope.^* 



It is commonly supposed that there is some special virtue in 

 formaldehyde that makes this the fixative of choice when tissues 

 are to be embedded in gelatine, but in fact almost any fixative may 

 be used. It is reasonable as a general rule to avoid those that 

 dissolve lipids. 



To prepare tissues for embedding in gelatine, it is only neces- 

 sary to wash out the fixative, generally with running water. If the 

 fixative was one that fixes gelatine as well as the proteins of the 

 tissues, it is important to get rid of it by thorough washing; for 

 otherwise it would fix the gelatine as it started to diffuse into the 



