Some Observations on Amoeba proteus. 195 
I watched, therefore, with great care the choice of diatom made by the 
amcebee—the favourites were Navicula, Nitschia, Synedra, Pinnularia, large 
and small. My next efforts were directed to the raising of “ persistent” 
cultures of these diatoms (fresh water replacing sea water), and following 
the lines laid down by Allen and Nelson! I was successful. 
The amcebe of the general proteus type were picked out from the pond 
supply, washed thoroughly several times in a sterilised solution in which 
the diatoms and amcebe flourished equally well. Each amceba was then 
placed in a sterilised watch-glass about half-full of the solution and supplied 
with a few diatoms from the “persistent” cultures. 
At this juncture the first radical difference among the several amcebee 
became evident—a difference casually remarked by many writers, but 
apparently only now beginning to attract attention. Certain amcebe fed 
voraciously on the diatoms and increased in numbers—while the others 
would not ingest a single diatom; this food idiosyncrasy was so great that 
no other food being available this latter “type” or “species” of A. proteus 
died rather than feed on the diatoms present. Other varieties of diatoms 
were tried, but always with the same result. When, however, the amceba 
was placed in stream water in which many small organisms abounded 
it would live and feed well on small flagellates, green algal cells, bacteria, 
etc., but not on diatoms. 
Upon the discovery of this differentiation in diet,? I revised my first 
opinion as to the unity of the members of the group and began to separate 
the amcebee into two categories, viz. the diatom feeders and the non-diatom 
feeders. These characteristics coincided so strangely at the time with the 
localities from which the amcebe were obtained that I classed the “types,” 
for my own convenience, ‘ Lapworth” and “Sutton” types—the Lapworth 
amocebe being the diatom feeders. | 
Other points of difference now became apparent in the nucleus and the 
crystals contained in the plasma. The Lapworth amcebe contained a 
spherical nucleus, as a rule, the nuclear membrane being completely filled 
with regular evenly-distributed chromatin granules. In the Sutton amebe 
the nucleus was variable, oval in outline, with varying arrangement of 
chromatin granules. The crystals? of the diatom-feeding amcebe were large, 
of the octahedral series, truncated at both ends of the long axis, also 
1 Loc. ett. 
2 A non-diatom feeder may indirectly ingest a diatom by ingesting small organisms 
which have themselves already ingested diatoms, e.g. small amcebe. 
% Leidy, loc. cit. There must be some mistake in the size of the crystals given on 
p. 47—‘075 mm. should surely be ‘0075 mm. A crystal of 75 «4 would be larger than 
the nucleus of the ainoeba, 
