728 MEDICAL ENTOMOLOGY 



insect or tick. Examine all the organs in rotation, at least in the 

 preliminary part of the work, and examine them separately. 



It is difficult to make proper smears of a gut which is full of blood, 

 and if circumstances permit it is better to wait until most of the blood 

 is digested. This applies especially to the examination of the gut of 

 Anopheles for the parasites of malaria. 



Eggs are also rather difficult to examine. The egg may be ruptured 

 with the point of a fine capillary pipette, and the fluid so obtained 

 blown on to a slide, fixing it at once ; or the egg may be smeared out in 

 the ordinary way. 



Coverslips are not satisfactory for ordinary use. Even when the 

 utmost care is taken, they get so brittle in the tropics that they become 

 impossible to handle with safety, and they have no advantage over slides 

 sufficient to compensate for this. 



Much has been written about the necessity for keeping the film wet 

 throughout all the fixing and staining process. The results obtained by 

 different observers are very discordant, and there is good reason to doubt 

 many of the statements made on the subject. Fixation with osmic acid 

 and absolute alcohol, drying, and the use of Romanowski's stain, are 

 sufficient as a routine practice. Special methods may be used to meet 

 particular circumstances, and for these the numerous papers on the 

 subject should be consulted. 



The osmic acid fixative is conveniently kept in a bottle the mouth of 

 which is just a little narrower than a slide. The upper surface of the 

 mouth should be ground down on a smooth stone, so that when the slide 

 is in position over the bottle the vapour will not escape. 



In the tropics the greatest care is necessary to keep absolute alcohol 

 anhydrous. Anhydrous copper sulphate, prepared by heating ordinary 

 blue stone till all the water of crystallization is driven off, should be kept 

 in the bottle, and replaced by fresh salt as soon as it acquires a 

 green tint. 



