727 



may adhere, and perhaps be broken as it is being detached. It is most 

 important to remove the isolated piece, and not to attempt any further 

 manipulations on the slide which contains other tissues. The piece may 

 be further examined at this stage with a high power and without a hair, 

 if required, but in this case it must be remembered that if parasites are 

 seen free from the tissue it may be only because they have been detached 

 by pressure. 



To prepare for staining proceed as follows. Using as small a 

 quantity of saline as it is possible to work with without the preparation 



becoming dry, open the organ with a fine needle, to 



n ^u rr 4 u Smearing out 



allow the organisms to escape. If they are extra- 

 cellular, the gut can then be removed with a needle, and the minute drop 

 containing them held at once over the mouth of the bottle containing 

 the osmic acid fixative. Fixation does not take more than a minute. 

 Then smear out the drop with a piece of coverglass broken to a conven- 

 ient size, endeavouring to get as small a smear as possible, so as to 

 have the parasites collected in one place. As soon as it is dry place the 

 slide in absolute alcohol for fifteen minutes, after which it is ready for 

 staining. 



The making of the smear must be done rapidly in the tropics, to 

 avoid the preparation getting dry before it is fixed. Pieces of broken 

 coverglass should be kept ready for use. 



If the whole of the organ is smeared out on the slide on which the 

 dissection was originally carried out the parasites get mixed up with the 

 tissue, and in the case of the gut also with the food, and not only 

 do they stain badly, but they are distributed over such a large area that 

 they may be difficult to find. It saves much time in the end to remove 

 the part required, and there is also this important advantage, that if 

 parasites are found one can be reasonably sure which part of the insect 

 they came from. 



If the parasite is intra-cellular, the tissue containing it must be teased 

 out under the dissecting microscope, and then smeared out with a 

 piece of coverslip. Often a hollow organ, such, for instance, as the 

 hind-gut of a fly or a diverticulum of a tick, can be spread out flat 

 and dragged along the slide in just a trace of saline till it is almost 

 flat. If fixed and stained in this position the wall of the organ is 

 thin enough to enable one to see the parasites through it. Endeavour 

 to use the natural juices of the tissue as much as possible, and a 

 minimum of saline. 



The examination should be by no means restricted to one part of the 



