56 PRACTICAL PABASITOLOGY 



should be watched, the process being complete when these have 

 stained a deep violet. The preparations should be well syringed with 

 distilled water, dried, and mounted in thickened cedar-wood oil 

 (immersion oil). Canada balsam should not be used or the stain will 

 not be permanent. On account of its convenience and the sharp 

 contrasts of colour which it presents, this method is invaluable for 

 preliminary examination and for diagnostic purposes. It is, how- 

 ever, useless for the study of minute details, the parasites becoming 

 to a large extent spoilt by the process of drying. For the latter 

 purpose : 



(2) Damp-fixed cover -glass preparations should be employed. 

 The method of preparing them has already been described (p. 9). 

 Still better is- 



(3) The preservation of blood in large quantities, by allowing it 

 to drip straight into a glass containing fixing mixture (alcoholic solu- 

 tion of mercuric chloride, Flemming's or Hermann's mixture). The 

 contents of the glass should immediately be centrifugalized, the fixing 

 mixture poured off, and the blood washed out. It is advisable to 

 centrifugalize after each change of fluid. Damp-fixed blood should be 

 coloured either with diluted haematoxylin, or with Salvin-Moore and 

 Breinl's modification of the iron-haematoxylin method, thus : The 

 preparations are allowed to remain for one hour in a mordant solution 

 of 3J per cent, iron-alum ; they are then stained for half an hour in 

 ^ per cent, solution of hsematoxylin in distilled water, to which, 

 after the solution of the colouring matter, a few drops of concentrated 

 watery solution of lithium carbonate have been added. Differentiate 

 with iron-alum in the usual manner. 



(b) DESCKIPTION OF CERTAIN MORE IMPORTANT FORMS. 



Although the blood parasites, as a class, are both numerous and 

 interesting, it is the purpose of the present work to describe those 

 forms only which are readily obtainable, and which are of practical 

 interest to the student. For further information, the special text- 

 books of the subject should be consulted. 



(a) Trypanosomidce. 



These parasites are free-living in serum. When mammalian blood 

 is not available, fresh material may be readily obtained from certain 

 fish. Crucians, tench and carp are the most frequent hosts. The 

 parasites are less prevalent in winter (November to February) than in 

 summer ; and they are more frequently found in second summer carp 

 than in first (Keysselitz). 



