36 THE AMEEICAN MONTHLY [February, 



The tissue should be placed for several hours, 24 or more, according 

 to its size, in an intermediate bath of a saturated solution of paraffine in 

 a fluid similar to that last used, as the change to fluids of different densi- 

 ties is thereby rendered less abrupt and liability to shrinkage is lessened. 

 It is well to stand the bottle of saturated solution of paraffine containing 

 the tissue on the top of the water bath used for embedding, where it 

 will be warmed and the penetration of paraffine solution furthered. 



III. Use only pure parafflne for embedding. 



The mixtures of paraffine and cocoa butter and the mixtures of wax 

 and oil have no advantage over the different grades of paraffine, but, on 

 the contrary, are decidedly inferior, as they are friable and lacking in 

 that cohesion which paraffine possesses, and which makes it such an 

 admirable support for delicate tissues during sectioning. 



Any embedding substance which crumbles ever so slightly will not 

 hold the tissue up to the edge of the knife with that firmness which 

 is essential to delicate sectioning, nor will it hold all parts in place 

 after the cut is made. By using paraflines of different melting points, 

 any degree of softness may be obtained, and this with more accuracy 

 than w^ith the mixtures sometimes advised. 



IV. When very thin sections are to be cut, embed in hard paraffine. 

 Thinner cuts can be made from a rigid substance than from a soft, 



yielding one, and this holds with paraffine as well as with other 

 materials. But hard paraffine, while it will admit of very thin section- 

 ing, will curl hopelessly when cut, unless the temperature of the room 

 is very high, thus practically reducing it to the condition of soft paraf- 

 fine, or unless the sections be kept flat by the use of a section flattener 

 of some form. For this latter purpose, after having tried most of the 

 mechanical contrivances recommended, I have come to the use of a 

 thin steel spatula held in the hand, and with its end catching the edge 

 of the section just as it begins to curl, thus preventing its rolling up. 

 By this means I find that I can keep large thin sections in better shape 

 than by the use of any flattener attached to the microtome. To be 

 sure, a certain degree of manual dexterity and steady hand are required, 

 but these are easily attained with a little practice, particularly if the 

 microtome be run by foot-power, so leaving both hands free. 



When embodying in celloidin, have the last solution very thick. 



If the celloidin solution is too thin, or the tissue has not been left in 

 a thick solution sufficiently long to insure its complete penetration, the 

 contraction of the celloidin when placed in diluted alcohol will distort 

 the tissue, and in delicate tissues will even produce rents in the weaker 

 parts. 



Sectioning. — I. If the microtome is of the sliding pattern, it is well 

 to run it by foot-power. 



This will leave both hands free to manipulate sections, and there 

 will be less liability of the knife jumping, as the force can be more 

 evenly applied than when the hand is used. I have my microtome 

 placed on an old sewing-machine table, and by a suitable arrangement 

 of cords and pulley-wheels a downward pressure of the foot on the pedal 

 causes the knife-carrier to run foi-ward, when, upon removal of the pres- 

 sure, the carrier is pulled back by a counter weight hung on a cord run 

 over a pulley-wheel at the back. 



II. A good " Wade and Butcher " razor is better than most section 

 knives. 



