254 The Philippine Journal of Science i9is 



dition arises — lack of resistance on the part of the host or some 

 other change in the host that intervenes to endow the parasite 

 with new powers, as has been hinted by Hadley — and the para- 

 site strikes boldly out into a new field, the tissues. What these 

 conditions are remains to be determined. That they can be ex- 

 plained on the same ground as we explain bacterial invasions 

 has been a popular supposition, which it seems to me would 

 be wise for us to abandon for the present at least and to look 

 for something new. We now know considerable about the re- 

 actions between animals and plants (bacteria), and it seems 

 about time we added to our knowledge regarding the reactions 

 between host animals and parasitic animals. 



Other points of inquiry bear on the discovery of the conditions 

 under which a parasite is harmless to its host when the latter 

 lives free but becomes pathogenic or even lethal when the host 

 is in captivity and of the possible free life of certain intestinal 

 protozoa as suggested by Rhamy and Metts, Escomel and 

 Smithies, and supported by laboratory experimentation. 



The time has arrived when workers in the field of parasitology 

 should fairly face the situation presented by the intestinal flagel- 

 lates. They have been under suspicion for many years, during 

 which time there has been gradually accumulating a mass of 

 evidence against them. No progress will be made if we are to 

 continue to employ, as our criteria of pathogenicity or non- 

 pathogenicity, the presence or absence of blood and pus in the 

 stools. Among the possible effects that may be produced by 

 these organisms may be mentioned : 



1. The production of antigrowth vitamines or growth-inhibit- 

 ing substances, as suggested by Gibson. 



2. The production of substances directly toxic. 



3. Unfavorable effects upon the host through the liberation of 

 the products of metabolism of the parasite. 



4. Mechanical irritation of mucous surfaces by the parasites 

 when present in large numbers. 



5. Interference with absorption in the intestine through the 

 adherence of large numbers of parasites to the surface of the 

 epithelium, as in the case of Lamblia. 



6. Actual invasion and destruction of the tissues with all its 

 concomitants and sequelae. 



Analysis of the work of Castellani on Entamoeba undulans, 

 of Gauducheau on Entamoeba and Trichomonas, and of Hadley 

 and others on the tissue-invasive power of Trichomonas and the 



