378 RESEARCH IN PROTOZOOLOGY 



gentle pressure during fixation. A very simple device for doing 

 this is made by cutting a triangle (Fig. 24) out of heavy paste- 

 board and inserting a bent pin at the tip of the acute angle. Rest 

 the point of the pin on the coverglass while applying the fixative. 

 The whole process is best done under the lens. After a few min- 

 utes' fixation under pressure, the slide may be set upright in a 

 fresh lot of fixative. 



When a considerable area of the specimen is in contact with both 

 slide and cover, the cover usually adheres during the whole proc- 

 ess of preparing these under-cover mounts. The thinner and 

 smaller the cover the better the chances of its adhering. If the 



Fig. 24. — Pasteboard triangle for holding down a coverglass during fixation. 



cover comes loose the specimen usually adheres to the slide. Slides 

 should be gently lowered into a Coplin jar in order to avoid jar- 

 ring the covers loose from them. 



All of the processes of fixing, washing, staining, dehydrating 

 and clearing under the cover must be greatly prolonged. The time 

 allowed for each medium must vary with conditions, the most im- 

 portant of which are the size of the cover and the distance between 

 the cover and the slide. One can safely pass from one medium to 

 another of much higher or lower concentration, since liquids dif- 

 fuse very slowly under the coverglass. There is little danger of 

 overstaining, so that one may leave the preparation over-night in 

 undiluted stains. When the specimen is in xylene, a small drop of 

 balsam is placed at the margin of the coverglass. As the xylene 

 evaporates the balsam replaces it. 



Very good preparations can be obtained by this method. The 

 specimen remains in the position in which it is first examined and 

 it may be examined under the high dry objective at any time dur- 

 ing the preparation. 



Fixatives. In general, the best fixative for mosquito stomachs or 

 salivary glands is that of Schaudinn, to which the acetic acid com- 

 ponent is added shortly before use. Bouin's is also a suitable 

 fixative. An hour's fixation will suffice for these small objects, but 



