PROTOZOA 381 



killed or paralyzed by heat ; that of the latter by cold. Or, it 

 may be, they do not find appropriate transmitters except in 

 special climatic conditions. Many of the protozoa acquire the 

 power of successfully invading the human body only after 

 certain developmental changes, which take place after they 

 leave their first host. Thus, according to Schaudinn, the 

 germ of amebic dysentery has to pass through a sporulating 

 stage before it becomes infective, and this stage is accomplished 

 outside of the body and in conditions of tropical heat. Hence, 

 amebic dysentery is a tropical disease. Other protozoan dis- 

 ease germs, notably those of malaria, yellow fever, trypano- 

 somiasis, and relapsing fever, require an animal intermediary 

 to remove them from the body of their original host, foster 

 them during a necessary stage of development, and reimplant 

 them in the human host. These animal intermediaries being 

 tropical, the diseases they disseminate are also necessarily 

 tropical." 



GYMNAMOEBIDA. 



Although in man a number of disease producing ameba 

 have been found, for example Ameba coli* but one species 

 seems to have been discovered pathogenic for domesticated 

 animals, namely Ameba meleagridis. 



AMEBA MELEAGRIDIS SMITH. 



This organism was discovered by Theobald Smith 6 in 

 1895 to be the cause of the very destructive disease of turkeys 

 popularly known as blackhead and described by him as infec- 

 tious enterohepatitis. Smith found this organism in the dis- 

 eased tissues of the caeca and liver. 



Morphology. The most frequent appearance of the or- 

 ganisms is that of round homogeneous bodies with a sharply 

 defined single contoured outline. They vary in size from 8 

 to 15 /x in diameter. The difference in size may be due to the 

 shrinkage caused by the fixing agents. In the tissues in the 



* Cause of dysentery in man. 



" Smith. Bulletin No. 8, B. A. I., U. S. Dept. of Agric., 1895. 



