270 TECHNIQUE 



is a coagulant of protoplasm, and its effect nuist be considered in 

 addition to the action of the fixative itself. It will be found that 

 the final picture obtained by different tem])eratures of the same 

 fixative and the same material will show certain differences, and 

 these modifications should l)e checked carefully. 



Records. — One of the most important things to remember is to 

 keep a careful record of the material, the organ or part of the organ 

 removed, the age and sex of the animal, the fixative used, the time 

 of fixation, and any modifying conditions, such as health of the 

 animal and method of killing. 



These details may })e kept in a note-book and corresponding code 

 numbers placed with the fixed material, or the entire information 

 may be recorded on the slip that accompanies the fixed material. 

 The writing on these slips should be done with a soft pencil. To 

 avoid confusion, do not place many tissues together in a bottle. 

 It is wiser to place not more than three small pieces in each bottle 

 so that no difficult^' later arises in identifying them for further 

 steps in the technique. 



PARAFFIN EMBEDDING. 



We have indicated the })roper length of time needed for fixation 

 with each of the fluids descril)ed abo^'e. It has also been shown 

 that the fixed tissue should be thoroughly washed free of all fluid 

 fixati^■e remaining. Usually the tissue pieces are preserved in 

 70 per cent (or NO per cent) alcohol. The next problem is to prepare 

 the material for cutting sections which after proper staining can 

 be studied microscoi)ically for their structural organization. Prior 

 to the development of modern technical processes, the early workers 

 at first held pieces of tissue or organ in one hand and cut thin slices 

 of it free hand with a sharp knife or razor. However, a piece of 

 fresh or fixed tissue "gi\'es" when sections are cut in this manner, 

 and the material is more or less crushed. This is avoided if the 

 tissue is infiltrated with paraffin. Paraffin is solid at room tem- 

 peratures, but a simple yet somewhat time-consuming technique 

 enables one to satisfactorily impregnate the tissue piece with ])araffin. 

 This is accomplished as follows: 



Paraffin is not miscible with water, so that all water nnist be 

 removed from the tissues. To accomplish this, the 70 per cent 

 alcohol in which the piece of tissue has been ke])t is discarded and 

 SO ])er cent alcohol replaces it. After an hour or so, the SO per 

 cent is discarded and replaced with 95 i)er cent alcohol. An hour 



