PREPARATIONS FROM THE CIRCULATING BLOOD 147 



the inoculations of the different tubes. In this manner the desired 

 dilution is obtained, and results in the formation of non-confluent 

 individual colonies. 



Another modification of the original plate method of Koch has been 

 devised by Esmarch; its details are the following: Tubes with fluid, 

 gelatin or agar, cooled down to near the point of solidification of the 

 medium, are inoculated successively as described above, but their 

 contents are not poured out. Instead the tubes are rolled on an ice 

 block until the medium has formed a solid coating on the inside of 

 the test-tube. The developing colonies can, as in the case of the plates 

 or Petri dishes, be examined under the microscope. 



FIG. 73 FIG. 74 



Method of holding tubes during inoculation. Esmarch's magnifier for counting colo- 



(McFarland.) nies of bacteria in Esmarch tubes. 



(McFarland.) ' 



Impression Preparation. When a plate or Petri dish has developed 

 colonies it is often desirable to study the finer details of the arrange- 

 ment of the bacteria forming the colony. This can be done by placing 

 a clean dry cover-glass over the colony and pressing it down tolerably 

 firmly on the culture soil. In this manner an impression of the colony 

 on the cover-glass is obtained. The latter is then lifted with a pair of 

 fine forceps, air dried, fixed, stained in the usual manner, and exam- 

 ined with the oil-immersion lens. The microscopic specimen obtained 

 in this manner is known as an impression preparation or "Klatsch- 

 Prseparat." 



Preparations from the Circulating Blood. It is often desirable or 

 necessary to make pure cultures from bacteria which may be present 

 in the circulating blood of a sick animal or person. This is done in 

 the following manner: The animal is first restrained. Next shave 

 and cleanse the skin so that it is sterile. Have on hand a sterile 

 (preferably all glass) syringe and a number of flasks containing from 



