BINUCLEATA 55 



it should be taken from the outer ear of dogs and the larger mam- 

 mals ; from the lobe of the ear of man rather than from the finger, as 

 is usually the case ; from a bare place on the breast of birds or, better 

 still, from the inner surface of the wing ; from the tail of tortoises ; 

 and from the back of fish. 



A movable stage should be used, as by this means the entire 

 specimen may be examined with the minimum of time and trouble. 

 Otherwise, there is little to add to the directions for the examination 

 of fresh material given in a previous chapter. These instructions 

 should be minutely followed, however, as nowhere is a scrupulous 

 attention to technique of greater importance than in the study of 

 blood parasites. If the growth of the parasites is not watched in the 

 living organism, but is merely deduced from dried and coloured speci- 

 mens, there is danger of developmental stages belonging to different 

 generations (schizonts and gametocytes of the malaria parasite) being 

 mistaken for the developmental stages of a single generation. Or, 

 again, a mixed infection may be overlooked and forms belonging to 

 widely different species may be mistaken for developmental stages of 

 a single parasitic variety. (As a matter of fact, both mistakes have 

 occurred.) 



Blood may be preserved in three ways : 



(1) Dry Cover-glass Preparations. The blood parasites are the 

 only Protozoa of which dry preparations can be made. Many fqrms 

 keep better if they are fixed by exposure to osmium vapour for five to 

 ten seconds before drying. They should be dried at room temperature 

 and not by passing them through a flame. To keep, dry preparations 

 should be wrapped in blotting paper and packed in tightly closed jars, 

 with a small quantity of calcium chloride to preserve them from damp. 

 They keep their colour somewhat longer in this way. Dry preparations 

 which have not been exposed to the action of osmium vapour should 

 be fixed before staining in 96 per cent, of absolute alcohol (time, ten 

 to twenty minutes) and again dried. They are best coloured by the 

 Giemsa modification of Romanowsky's stain. Giemsa solution (which 

 is supplied ready for use by Griibler and Co., Leipzig) is employed in the 

 proportion of one drop to each cubic centimetre of distilled water, 

 and the mixture should be used immediately. The preparations must 

 be placed material downwards in the stain, or they will be spoilt by 

 precipitates. Two specimens may be stained at the same time, if one 

 cover-glass rests with all four corners touching the bottom of the 

 watch-glass while the other is made to float upon the surface of the 

 fluid. The specimens will, as a rule, be sufficiently coloured in fifteen 

 to twenty minutes, but the process should be watched under the 

 microscope. When finished, the plasma should be stained blue and the 

 nuclei red. If the parasites are very scanty, the blood corpuscles 



