724 MEDICAL ENTOMOLOGY 



paraffin does not penetrate. The margin between the two extremes is very 

 small, and has to be determined by experiment for each particular tissue. 

 For this reason it is advisable to start with as many specimens of the 

 same material as possible, and to give each a different length of time in 

 the bath, noting the time given to the best sections for future experiments. 

 The results are not very satisfactory at the best, and it is seldom that 

 one can obtain a perfect series of sections of a whole insect or tick. 

 For practical purposes all the information required can usually be 

 obtained by sectioning the parts separately. 



Advantage should, whenever possible, be taken of the fact that the 

 chitin of a newly emerged insect is soft and cuts well ; though reagents 

 will not penetrate it at all readily. 



Perhaps the most important detail in the whole technique of cutting 

 sections is the sharpening of the razor. Even the best razors are of 

 little use as they come from the maker, the edge being 

 much too abrupt and wedge-shaped. They should be 

 ground down on a soft stone, lubricated with a plentiful supply of fil- 

 tered petroleum. It is convenient to fit up a burette over the stone, 

 so adjusted that a constant trickle of oil falls from the tap, the oil 

 being poured through filter paper in a funnel at the open end of the tube. 

 The old edge of the razor comes off in shreds, which must be carefully 

 removed from the stone, or they will spoil the new one. When the razor 

 presents a perfectly smooth and unbroken edge under the- microscope, 

 and will cut a hair longitudinally, it should be finished off on a strop, 

 first with razor paste and then without. If one portion of the blade 

 gets chipped in cutting a hard part, the razor can be moved a little to 

 one side in the holder, so that a new portion comes into use ; by doing 

 this a single razor can be used through a series without further stropping. 

 It may take a whole day to sharpen a razor properly. 



In order to make the sections adhere to the slide a fixative composed 

 of equal parts of glycerine and white of egg is used ; a very small 

 quantity of this being smeared over the slide. This makes the slide 

 sticky, and some care is necessary in placing the rows of sections on it, 

 for once they are laid down, it is not easy to rearrange them. 



For straightening out the sections and making them adhere to the 

 slide the following procedure is at once quick and effective. A small 



Stretching and l uan tity of water, freshly boiled to expel the air, is 



mounting sections P^ced on the slide with a fine bore pipette, and allowed 



to flow all over till all the sections are wet. The 



.copper plate used for embedding is previously heated up to about the 



