DIENTAMOEBA FKAGILIS I 23 



Each individual is typically binucleate (figs. 90, 91, PI. V),the two nuclei 

 being identical in size and structure. Their size is proportional to that 

 of the whole organism, and their diameters range from about o"8 fi in very 

 small amoebae up to about 2*3 yu. in very large. On the average the 

 nuclei measure about 2 yx, in stained specimens. They are invisible or 

 inconspicuous in the living organism ; but in well fixed and stained indi- 

 viduals, obtained immediately after they have left the human body, they 

 have a very characteristic structure — unlike that of the nuclei of any of 

 the other intestinal amoebae of man. Each nucleus is spherical and 

 vesicular. The chromatin is all situated in a fairly large central 

 karyosome, which, in well-differentiated iron-haematoxylin preparations, 

 can be seen to consist of a number of granules apparently embedded 

 in a plastin matrix (see figs. 90, 91). The granules are not always 

 unifoiin in size, and one of them is often larger and more conspicuous 

 than the others (cf. the lower of the two nuclei in the amoeba depicted 

 in fig. 91, PI. V). No centriole is demonstrable in the karyosome. The 

 whole nucleus is bounded externally by a very delicate and feebly 

 staining nuclear membrane, separated by a clear zone from the karyo- 

 some. Linin threads of extreme tenuity can be seen, in well stained 

 individuals, radiating from the kar3'osome to the nuclear membrane. As 

 a rule, excessively minute granules can be resolved on or in the nuclear 

 membrane — usually at the points where the linin threads are attached 

 to it. Apart from these granules, of doubtful composition, there is no 

 "peripheral chromatin" either in the clear zone outside the karyosome 

 or on the nuclear membrane. The structure of the nuclei will be evident 

 from inspection of ligs. 90-92 (PI. V). 



The two nuclei of a Dientamoeba may occupy any position, relatively 

 to one another, in the body of the organism. They are sometimes in 

 contact (fig. 91), or may be separated by an interval of variable extent 

 (fig. 90). 



Most individuals are binucleate, but in every infection a certain 

 number of uninucleate specimens can always be found (fig. 92). Careful 

 examination of over 1,000 individuals from three different cases showed 

 that about 80 per cent, of them were binucleate and 20 per cent, 

 uninucleate. Uninucleate individuals may be found of all sizes, from 

 the smallest to very large ; but they are, on the whole, somewhat smaller 

 than the binucleate forms. These, however, are sometimes of extremely 

 small size (3*5 fi). 



Division stages are extremely rare in the stools, and all that have so far 

 been found represent organisms with a single dividing nucleus. From this 

 fact — and others just noted — I have been led to suppose that D. fragilis 

 differs from the other known binucleate species of amoebae (such as 

 "Amoeba" binucleata Gruber, and ^'Amoeba" diploidea Hartmann et 

 Nagler) in its mode of reproduction. It seems probable that the organism 

 is, when full grown, binucleate; and that when division occurs, it con- 

 sists in a simple fission of the cytoplasm, resulting in the formation of 

 two uninucleate daughter individuals. The nucleus in each of these 

 divides into two during the growth period, thus giving rise to the adult 

 binucleate form once more. 



D. fragilis undergoes degeneration in a very characteristic manner. 

 Soon after leaving the body the amoebae usually become rounded and 

 motionless, and filled with vacuoles. These then coalesce into a single 

 large central vacuole, surrounded by a thin layer of protoplasm. The 



