EMBEDDING 75 



commercial products are mixtures of molecules of different 

 molecular weights. The waxes commonly used in microtechnique 

 melt at various temperatures between 52° and 60° C. This sug- 

 gests that the number of carbon atoms in most of the molecules 

 is less than 30, for H3C(CHo)28CH3 melts at 66° C. 



Paraffin wax should be maintained at a temperature only a 

 few degrees C above its melting-point. A wax of high melting- 

 point (that is, with long molecules) should be chosen if thin 

 sections are required, or if the microtome is to be used in a warm 

 room. There is no advantage in transferring tissues first to a wax 

 of low melting-point and then to one of high. Indeed, the wax of 

 low melting-point would be ousted from the tissue by the other 

 wax very slowly and probably incompletely, for these large 

 molecules replace one another slowly by diffusion. 



When the tissue is placed in melted paraffin, the antemedium 

 passes out to mix with it, while the separate paraffin molecules 

 diffuse in to replace them. Great shrinkage of the tissue will occur 

 if the antemedium escapes much more quickly than it can be 

 replaced. If, on the contrary, it escapes too slowly (because it is 

 insufficiently soluble in or miscible with paraffin), replacement 

 is likely to be incomplete. Toluene possesses neither of these 

 defects. 



It is best to change the melted paraffin once or twice, in order 

 to get rid of the antemedium that has escaped from the tissue and 

 thus to facilitate the escape of the remainder and its replacement 

 by paraffin. 



Tissues are much more fully permeated by paraffin than by 

 gelatine. The embedding medium enters the cells, and indeed 

 there is proof that it sometimes penetrates their nuclei: for 

 paraffin crystals, being birefringent, advertise their presence when 

 sections are examined in polarized light. ^^^ Penetration takes 

 place readily if the proteins of the cells have been coagulated, but 

 with difficulty if they have been fixed homogeneously. It is partly 

 for this reason, in all probability, that coagulant fixatives have 

 retained their popularity, despite the fact that they necessarily 

 distort the finer structure of the cell. Protoplasm that has been 

 fixed by osmium tetroxide is scarcely porous, and paraffin 



