338 H. S. DAVIS 



ters to begin with the small, mono-nuclear trophozoites. This 

 stage occurs only rarely in my material, and is characterized by 

 the relatively large amount of dense, finely granular cytoplasm 

 containing a large, well defined nucleus (figs. 8 and 9). Within 

 the nucleus the chromatin forms a well defined network com- 

 posed of irregular granules varying greatly in size and especially 

 abundant around the periphery. A conspicuous nucleolus is 

 always present and often (especially in strongly decolorized 

 specimens) can be seen to be composed of a deeply staining 

 outer portion enclosing a central mass which stains less intensely. 

 Usually — possibly in all cases — the nucleolus is attached to 

 the nuclear membrane, and sometimes, as in figure 9, the side 

 in contact with the membrane is somewhat separated from the 

 rest with which it is connected by a faintly staining substance. 

 The next stage in the development of the trophozoites is shown 

 in figures 10 to 15. The trophozoite now contains two nuclei of 

 equal size and similar appearance, each surrounded by a layer 

 of dense cytoplasm which, in most cases, can be more or less 

 clearly differentiated from the less deeply staining material com- 

 posing the remainder of the individual. The denser, more deeply 

 staining area of cytoplasm around each nucleus is, however, 

 apparently not surrounded by a definite membrane at this time. 

 While it can usually be more or less easily distinguished from 

 the surrounding cytoplasm, in some cases the two merge to- 

 gether so gradually that no distinction can be made (figs. 12 and 

 16). That even in such cases there is a specially differentiated 

 area around each nucleus is clearly shown in figure 26, where, 

 owing to a defect in fixation, the cytoplasm immediately sur- 

 rounding the nucleus has shrunken away from the adjacent 

 endoplasm. These two nuclei are, I believe, derived by mitotic 

 division from the single nucleus of the preceding stage. In 

 a dried smear stained by Giemsa's method, I found a mono- 

 nuclear trophozoite in which the nucleus was dividing mitotically. 

 Moreover, the bi-nucleate stage is often scarcely, if any larger, 

 than trophozoites which contain but a single nucleus and thus 

 can scarcely be formed by the fusion of two such trophozoites. 

 Although the two nuclei are similar in size and appearance, yet 



