REPRODUCTION AND THE LIFE CYCLE 195 



in keeping up the various races of parasites, and infection of new hosts 

 may be brought about by (a) breathing; (b) by direct transmission or 

 contact; (c) by inheritance; or (d) by indirect transmission through 

 the agency of intermediate hosts. 



(a) Air-borne Protozoan Parasites. — So far as the protozoa are con- 

 cerned, this method of infection plays but httle part, and then only in 

 cases of certain diseases, such as scarlet fever, smallpox, and a few 

 others which are not yet accepted by all as due to protozoan parasites. 

 The great majority of protozoa capable of withstanding the condition 

 necessary for this mode of infection are too large and heavy to be 

 conveyed as dust. In trachoma, smallpox, and scarlet fever, which no 

 one would question as being germ diseases, the spores of the organ- 

 ism causing them are so minute as to be readily disseminated with 

 cutaneous debris, or as Fliigge ('97) has shown in experiments with 

 bacteria of different kinds, they may be spread in minute droplets of 

 mucus or sputum. So far as known, the seat of invasion of these 

 spores or minute organisms is the respiratory tract, where the nasal 

 lining may harbor the spores of trachoma, or the corrugated surface 

 and imperfect epithelium of the tonsils may give lodgement for the 

 spores of smallpox and scarlet fever. It is possible that the organism 

 found by Minchin and Fantham ('05) in nasal tumors (Rhinospori- 

 dium hmealyi) is transmitted in this way, although nothing is known 

 as to the exact method of its dissemination. 



(h) Transmission of Protozoan Parasites by Contact. — A large number 

 of protozoan diseases are due to the transmission of the parasites by 

 direct transmission through contact which may be brought about in 

 various ways. Wherever external lesions occur this means of infection 

 is possible. In the case of rabies, where contact is brought about 

 usually by the bite of some infected animal, the parasites are intro- 

 duced with the saliva and gradually make their way into the central 

 nervous system, although, as Pasteur first showed, the entire nervous 

 system from periphery to centre may contain the virus. Not only by 

 biting, but by other ways as well, may the organism of hydrophobia 

 (Neuroryctes hydrophobia:) get into the human organism; infection 

 may follow from carelessness in the operating room, or, a particularly 

 potent way, from the licking of infected animals on abraded or chapped 

 surfaces of hands or face. 



Usually the organisms thus transmitted by contact have the power of 

 spontaneous motion, the passive sporozoa being rarely spread in this 

 way. A possible exception, however, appears to be the case of the 

 so-called Coccidioides imtnitis, described by Rixford and Gilchrist 

 ('97), in Argentina and the Southern States. The disease first mani- 

 fests itself in the human skin, and may pass by way of the lymphatics 

 to liver, spleen, peritoneum, and other organs of the body, ultimately 

 causing death. The organisms first form small granulation tumors in 



