PROTOZOA 137 



curves of certain spirochaetes lie in one plane and, consequently, that 

 their bodies are really waved and not spiral. These organisms present 

 no organized nucleus but the chromatin appears to be distributed 

 throughout their bodies. 



Those parasites which are important enough to require special con- 

 sideration aredescribed (page 822) in the order in which they are men- 

 tioned in the classification (page 13). Whenever it is possible to do so, 

 a single species is taken as the type of each genus and that species, with 

 the disease it produces, is described; if the remaining species of the 

 genus are mentioned, they are spoken of only to indicate how they 

 differ from the description of the type species. 



Technic* 



The methods employed in studying the pathogenic protozoa are very similar to 

 those used in bacteriology. Microscopes, with the highest magnifications, are 

 essential for successful work. 



It is of great importance in the study of protozoa to examine these organisms in 

 the living condition. In no other way can their mode of locomotion be determined 

 and frequently their contour is also quite different in life and in stained preparations. 

 A small amount of the material in which they occur may be placed beneath a cover- 

 glass on a clean slide and examined immediately with the microscope by ordinary 

 daylight. In case large organisms are examined in rather thin fluid it is well to 

 prevent their being crushed by interposing several minute globules of paraifin 

 between slide and cover-glass which is readily accomplished by touching parafiSn 

 with a hot needle and transferring it thus melted to several points on the slide before 

 the preparation is made. When very minute forms are to be studied it is necessary 

 to utilize what is known as the dark field illumination. This brings out very minute 

 organisms and particles which being transparent are invisible to ordinary transmitted 

 light. The dark field apparatus consists of a strong source of light such as a small 

 arc lamp, a special condenser which deflects the light so that objects in the micro- 

 scopic field are illuminated by light directed from the sides causing them to appear 

 bright on a dark background. Another method of obtaining a dark field is to mix 

 on a slide a small drop of the material to be examined with an equal-sized drop of 

 India ink or better of saturated aqueous solution of nigrosin and then to smear this 

 mixture across the surface of the slide when it may then be dried and examined at 

 once by the oil immersion lens. Only ordinary daylight is required for this method 

 but it does not serve in the study of the motility of organisms. 



By special apparatus it is possible after obtaining, a certain amount of skill to 

 dissect many forms of protozoa. In this way knowledge is obtained of the physical 



• For a more extensive treatise of the technic applicable to the study of protozoa see Doflein, 

 Lehrbuch der Protozoenkunde, Jena, Gustav Fischer; Prowazek, Der jnikrochopischen Technik 

 der Protistenuntersuchung, Leipzig; and Stitt, Practical Bacteriology, Bloodwork and Para- 

 sitology, Blakiston, Philadelphia. 



