576 PROTOZOA 



purposes should be carried out as follows (Wenyon, Lancet, ii, 

 1915, p. 1173) : From a suitable part of the specimen sent to the 

 laboratory take a minute portion on a platinum loop and rub it 

 into first a small drop of normal saline and then one of iodine 

 solution on slides and cover each with a coverslip. It is necessary 

 to examine several preparations from different regions of a 

 specimen before it can be diagnosed as free from any special 

 protozoa, 



Osniic Acid. One per cent, solution may be used to kill and 

 fix protozoa while actually under observation, and may be followed 

 by such a stain as picro-carmine. 



Methyl green acetic acid mixture may also be allowed to diffuse 

 into a preparation while under observation. 



A saturated solution of methyl green in 0-5 to 1 per cent, of 

 acetic acid is an important stain for showing up the nuclei of 

 protozoa. It is a chromatin stain for fresh (unfixed) cells. Refer 

 also to §§ 621, 623. 



FIXING AND PRESERVING FOR PERMANENT PREPARATIONS 



1126. Films and Smears. When films are mentioned in the 

 following notes, it is to he understood that a thin layer or smear of 

 the material on a coverslip is meant. The making of these requires 

 much practice and care. The protozoa or other material must 

 be sufficiently spread out to give complete transparency, and the 

 films must never be allowed to dry, for then the protozoa would 

 be distorted ; on the other hand, if too wet, most of the material 

 will wash off in the fixative.* When just sufficiently dry, the 

 coverslip is dropped, film downwards, on to the surface of the 

 fixative already prepared in a watch-glass. Minchin {Quart. 

 Journ. Micr. Sci., liii, 1909, p. 755) elaborated a process of dealing 

 with these films by passing them through the different reagents in 

 solid watch-glasses or capsules. The upper side of the cover- 

 glass is kept clean throughout so that at any stage the process 

 can be controlled by examination under the microscope. Curved 

 forceps are useful for transferring the coverslips from one watch- 

 glass to another ; since they float on water, the films can be easily 

 washed in running water, if desired. The other reagents are best 

 introduced by means of a pipette underneath the coverslip 

 while its corners rest on the watch-glass. 



For most protozoa, this is the best way to deal with them 

 whole, but for blood parasites, a rougher method may be used as 

 well, by spreading out the blood on slides. These preparations. 



* In making films of free-living flagellates and ciliates it is generally 

 necessary to add a trace of gelatin water or Mayer's albumen (§ 209) 

 to help them to adhere to the cover-glass. With parasites there is 

 usually enough albumen from the host fluids to answer the purpose. 



