686 INFECTION AND IMMUNITY. 



recovered, a milder infection results than when the blood 

 of an actively infected animal is used (Pound, cited by 

 Kossel ) . The resulting immunity is not an absolute one, 

 however, and the percentage of mortality is fairly high. 

 .According to Dodson, the serum of animals which have 

 completely recovered has no protective power for other 

 animals. 



For prophylaxis it is important to free the cattle 

 from ticks (as by an oil bath) and to avoid infected 

 fields. If cattle are kept from an infected pasture for 

 two years, the ticks die out very largely ( Morgan ) . 



IV. AMEBIC DYSENTERY. 



Ameim. Amebae are unicellular animal organisms which 

 contain one or more nuclei, a "contractile" vacuole, 

 a granular endoplasm and a tougher more hyaline 

 ectoplasm, having the power of locomotion by 

 means of pseudopodia or by a gradual flowing for- 

 ward of the cytoplasm. They nourish themselves 

 by digesting bacteria and other lower organisms or 

 solid particles of decaying matter, which they in- 

 gest after the manner of phagocytes. They pro- 

 liferate by division of an adult cell into two daugh- 

 ter cells, and certain of them reach a cystic stage 

 in which hundreds of endospores are formed 

 (Amoeba proteus). Some of them utilize higher 

 animals as hosts only occasionally, while others 

 are known only as parasites. They frequently are 

 encountered in the intestines of mice, frogs and 

 other animals. 



Distribution. AmebaB are widely distributed in nature, exist- 

 ing to the depth of 2 meters in tropical soils, in 

 the water of springs and wells and practically all 

 surface waters (hot countries), and in stagnant or 

 sluggish waters in higher altitudes. They exist on 

 hay, fruits and vegetables of all kinds, especially 



