PROTOZOA 511 



similar to those used in Bacteriology, are well referred to as smears 

 to distinguish them from films on eovcrslips. They may be 

 allowed to dry during fixation (see Osniie Vapour, § 1144, below), 

 and are well stained with Giemsa, §1155. The drying eauses the 

 protozoa to flatten out somewhat and therefore to appear rather 

 larger, whieh may be a great advantage in dealing with small 

 flagellates and their flagella, so long as distortion is not too great. 

 Wet fixation, on the other hand, always tends to produce shrinkage 

 but gives a more reliable representation of the organism. 



1127. Free Protozoa. The older method of treating protozoa 

 while mounted between coverslip and slide is still sometimes 

 used, especially for free-living forms — the various reagents being 

 run in under the coverslip by means of pipettes and filter paper 

 (Mixciiix, Nat. Sc, iii, 1893, p. 112 ; Doflein, 1916, p. 376). 



\Yhen the protozoa are abundant they may, of course, be 

 treated en masse by pouring into comparatively large volumes of 

 fixative in a capsule or centrifuge tube and allowing them to 

 settle. The subsequent processes of washing, staining, and 

 dehydrating may be hastened by centrifuging slowly. To clear, 

 pour clove oil down the side of the tube containing the protozoa 

 in absolute alcohol. They will soon sink to the bottom, and 

 can be taken up with a wide pipette and distributed on slides as 

 required, and mounted (Dofi.ein, p. 380). 



1128. Sections. When protozoan parasites in tissues are to be 

 dealt with, it is necessary to determine the true relationship of 

 parasite to host by fixing with as little disturbance as possible 

 small pieces of the tissue taken from the host immediately after 

 death. The tissue is then imbedded for sections. 



1129. Imbedding Protozoa (Minciiin, Quart. Journ. Micr. Sci., 

 Ix, 1915, p. 508). A thin slice of a block of amyloid liver, pre- 

 served in alcohol, is placed in a shallow glass vessel with a flat 

 bottom, containing alcohol to the height of about 1 cm. The 

 dish is placed on the stage of a dissecting microscope. The 

 objects to be imbedded are taken up in a pi})ette from 90 per cent, 

 alcohol, after fixation, and placed a few at a time on the slice of 

 liver and orientated as desired. A tiny drop of glycerin albumen 

 solution is taken up on the point of a needle and caused to touch 

 the surface of the alcohol immediately above the small objects. 

 The dense albumen solution falls at once through the alcohol and 

 spreads out over the objects on the liver ; at the same time the 

 glycerin is extracted and the albumen coagulated by the alcohol, 

 with the result that the objects are stuck on to the liver. \Vhen a 

 sufficient number has been attached in this way the piece of liver 

 is trimmed, if necessary, and imbedded in the usual way. See 

 also § 157 for other methods. 



For the ordinary procedure of dehydrating, clearing, imbedding, 

 etc., reference may be made to p. 723 for students. 



VADE-MECUM. 19 



