MAKING SECTIONS 55 



Dehydrating. The classic method of dehydration is to soak the object in a 

 graded series of alcohols, usually 10 or 15 per cent apart. Dehydration through 

 gradually increasing strengths of alcohol may be vital when one is dealing 

 with delicate objects containing easily collapsible cavities, such as chick and 

 pig embryos, but a block of tissue may be taken from water to 96 per cent 

 alcohol without any appreciable damage. Even if one uses increasing strengths 

 of alcohol, the series normally in employment at the present time is by no 

 means satisfactory. It is customary, for example, to pass the object from water 

 to 30 per cent alcohol at one end of the series and to pass it from 85 per cent 

 to 96 per cent at the other end of the series. An intelligently graded series for 

 delicate objects should run from water to 15 per cent alcohol to 40 per cent 

 alcohol to 75 per cent alcohol to 96 per cent alcohol rather than through the 

 conventionally spaced gradations. This is not at all in accordance with the 

 recommendations in most textbooks but is based on the author's experience 

 over a long period of time. In using this classic method of dehydration, it is 

 not necessary to confine the technique to ethyl alcohol. Methyl alcohol or 

 acetone will dehydrate just as effectively although they are more volatile. 



The substitution of a solvent which is miscible both with water and molten 

 wax for a straight dehydrating agent is in vogue today. The best-known of 

 these solvents is dioxane, though ;z-butyl alcohol has also been recommended. 

 The author is not completely satisfied with these methods because, although 

 the solvents invo'ved are excellent dehydrating agents, they are relatively poor 

 solvents of paraffin and frequently occasion great shrinkage of delicate objects 

 in the final transition between the solvent and the wax. For such objects as the 

 routine examination of the tissue blocks in a pathological laboratory or the 

 sectioning of relatively sturdy plant materials, they may justifiably be em- 

 ployed. However, for sections in which structures are to be retained intact for 

 subsequent research, it is to be recommended most strongly that the standard 

 routine of passing from a dehydrating to a clearing agent be followed. 



Clearing. The choice of a clearing agent in section cutting is of far more 

 importance than the choice of a dehydrant, since there is not the slightest 

 doubt that prolonged immersion in xylene leads to a hardening of the tissue 

 with subsequent difficulty in sectioning. Benzene is much to be preferred for 

 most objects. 



It is still recommended occasionally that essential oils, such as cedar oil, be 

 used for clearing objects for embedding. There is no justification for this unless 

 it is vital that the object be rendered transparent rather than alcohol free, in 

 order that some feature of its internal anatomy may be oriented in relation to 

 the knife. Essential oils are excellent for wholemounts but they are not readily 



