Making Sections 129 



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 employ- 

 ment 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 con- 

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

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

 ence over a long period of time. In using this classic method of dehydra- 

 tion, 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 that 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 n-butyl alcohol has also been 

 recommended. The author is not completely satisfied with these methods 

 because, although the solvents involved 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 employed. 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 whole- 

 mounts but they are not readily removed from the specimen by molten 

 wax, so that if they must be used they should always be washed out with 

 a hvdrocarbon before the wax bath. Relativelv small traces of any essential 

 oil will destroy the excellent cutting properties of any wax mixture and. 



