140 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



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

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

 no organized nucleus. The chromatin is distributed throughout their 

 bodies. 



Those parasites which are important enough to require special con- 

 sideration are described (page 876) 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. 



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 them in the living 

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

 frequently their contour is quite different in living and in fixed 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 paraffin 

 between slide and cover-glass. This is readily accomplished by touching paraffin 

 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 trans- 

 mitted 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 

 microscopic 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. It is then dried and examined at 



*For more detailed instructions for the study of protozoa see Fantham, Stephens and 

 Theobald, The Animal Parasites of Man, William Wood & Company, New York; Castellani 

 and Chalmers, Manual of Tropical Medicine, Bailliere, Tindall & Cox, London; Stitt, Practical 

 Bacteriology, Blood Work, Parasitology, Blakiston, Philadelphia; Brumpt, Precis de Parasit- 

 ologie, Masson, Paris; Langeron, Precis de Microscopic, Masson, Paris; Doflein, Lehrbuch der 

 Protozoenkunde, Gustav Fischer, Jena; and Prowazek, Der mikroskopischen Technik der 

 Protistenuntersuchung, Leipzig. 



