=Etiology.=—For a long time the authorities in Manila held that it was impracticable to differentiate between a pathogenic and nonpathogenic species, taking the view that the principal factor in the production of dysentery was that of symbiosis between amoebae and suitable bacteria, it having been thought that they observed in cultures of amoebae evidences of both symbiosis and antagonism on the part of amoebae to certain species of bacteria. They furthermore were convinced that pathogenic amoebae could be cultured on a medium of about 1/10th the strength of ordinary nutrient bouillon or agar and that dysentery could be produced by such cultural amoebae. Such views had an important bearing on epidemiology as it was thought that where amoebae could be cultured from green vegetables, fruit, or water supply there was positive evidence of the possibility of infection with amoebic dysentery from such a source. The above views are no longer entertained and, due to Walker, working in Manila with experiments on man, we now know that cultural amoebae are without effect in the production of dysentery and that there are certainly two well known species of amoebae having man for a host, the one pathogenic, _Entamoeba histolytica_, and the other a harmless commensal, _Entamoeba coli_. Two other species, _Endolimax nana_ and _Iodamoeba bütschlii_, have recently been described but are generally considered nonpathogenic. Some authorities prefer the generic names _Löschia_ and _Endamoeba_ to _Entamoeba_. Schaudinn, in 1903, described the pathogenic amoeba, which he named _E. histolytica_, as follows: 1. Distinct, highly refractile and tenacious ectoplasm. He considered this tough external portion of the cytoplasm as the explanation of the ability of the pathogenic amoeba to bore its way into the intestinal submucosa. 2. Eccentric nucleus which was indistinct by reason of little chromatin. 3. Reproduction by peripheral budding in which small aggregations of chromatin reached the periphery of the cytoplasm and, enclosed in a resistant capsule, broke off from the parent amoeba and constituted the infecting stage. [Illustration: FIG. 59.—_Entamoeba histolytica_. The same living individual drawn at brief intervals while moving. (From Doflein after Hartmann.)] For the nonpathogenic _E. coli_ he noted: (1) No distinction between a granular endoplasm and refractile ectoplasm; (2) centrally placed and sharply outlined nucleus, rich in chromatin; and (3) encystment with the formation of eight nuclei, which cysts with their nuclei or amoebulae form the infecting stage.