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THE AMERICAN NATURALIST [Vol. LI 



What is the first stimulus that creates out of a commonly 

 law-abiding protozoan, an invader that has no equal 

 among protozoan forms in the rapidity and completeness 

 with which it carries on its ravages in the intestinal 

 tissues? 



This is a difficult question, and one which can not be 

 answered with any degree of finality at the present time. 

 The facts of the matter are these: The manifestation of 

 the disease, as it appears for instance in the so-called 

 blackhead of turkeys, is invariably preceded by a diarrheal 

 condition in which the flagellates appear in increasing 

 numbers as the course of the disease advances. Finally 

 they appear, not only in the liquid cecal content, but in 

 the very depths of the cecal tubules or crypts; and finally 

 in the tissues behind the epithelial wall. From this posi- 

 tion, by a process of autogamous reproduction, the in- 

 vasion of the mucosa, submucosa, muscularis mucosas 

 and even the muscular layers, goes on rapidly ; and even- 

 tually the whole cecal wall is crowded with the parasites. 

 Secondary bacterial infections may intervene and the re- 

 sults are almost invariably fatal. The question now 

 arises : Are these countless flagellates, present in the liquid 

 cecal contents at the beginning of the attack, the cause or 

 the result of the diarrheal condition? Clinical evidence, 

 which can not now be presented in detail, seems to indi- 

 cate that the latter circumstance is the actuality : that the 

 diarrhea is the primary condition and the increase in the 

 number of parasites the secondary. To explain the first 

 cause" of the disease, then, one must explain the cause 

 of the diarrheal condition; and this, of course, is likely 

 to prove in itself, a complex problem, but seems to lead 

 back to certain circumstances related to the nature of the 

 food materials and their assimilation, lying outside the 

 province of the present paper. 



For n lonsr time it was not clear how, after their rapid 

 miilti))li('ntion in the intestinal content, the parasites were 

 al)le to |ieuotrate the epithelial wall and reach the sub- 

 epitlH'linl ti-<no^. Rocont studie^^ liave shown the role 



