METHOD OF PRESERVING SPECIMENS 99 



according to the species to be examined and the purpose for which 

 the specimen is required. 



(a) METHOD OF PRESERVING SPECIMENS WHOLE. 



(1) Trematodes. Flat worms, of slight muscular structure, are 

 best for preservation whole, while very large and thick round worms 

 are unsuited to the purpose. The specimens should be washed in 

 normal saline solution immediately after their removal from the host ; 

 they should be freed from mucus, &c., with a brush, and then stretched 

 out upon a glass slide. A cover-glass of sufficient size and thickness 

 should be smeared with a drop of the fixing fluid and inverted on 

 to the worm, which, as a rule, remains stretched out on the slide, 

 without bending or folding, and becomes flattened out. This flatten- 

 ing process may be lessened or increased according to the amount of 

 fluid introduced under the cover-glass. It should be added drop by 

 drop at one edge, while the superfluous fluid is drawn off by means 

 of filter paper from the opposite edge. The worm should not be 

 crushed, nor should it be flattened to such an extent as to change the 

 shape of the organs. The expulsion of ova from the uterus is a sign 

 that the pressure is considerable, while the passage of faeces from the 

 oral sucker points to a still higher degree of pressure. It is a good plan 

 to arrange supports by the side of the object to be examined, and 

 these may be cut from writing paper, blotting paper, post-cards, or 

 cartridge paper, in accordance with the thickness of the parasite. 

 The strips should be quite narrow and should be laid upon the glass 

 slide in such a way that, when the cover-glass is applied, they may be 

 under it. 



There are certain varieties, even among the smaller worms, in 

 which muscular action is strong, and it will be found that the pressure 

 of the cover-glass alone, even when increased by the withdrawal of 

 the fluid, is insufficient to keep these varieties flattened out. In such 

 cases a glass slide, or small pieces of lead or iron, should be put upon 

 the cover-glass ; or it may even be necessary to use a press. Extra 

 pressure is indispensable when preparing large flatworms. 



The best fixing fluid is undoubtedly a formula used by D. Hofer. I 

 Fifty parts of a saturated watery solution of picric acid, are mixed ' 

 with 48 parts of water, and 2 parts of glacial acetic acid. The mixture 

 acts first upon the edges of the specimen and works inwards very 

 rapidly, but it is sometimes unable to penetrate to the two surfaces 

 touching the glass. In that case, the unaffected parts will remain 

 transparent while the edges will be yellow and opaque. The defect 

 may be remedied by gently raising the cover-glass and allowing it 

 to fall back again into place. It may, however, be necessary to add 



