The Spirostoma used in this study were obtained from testtube cultures made with boiled wheat-grains and boiled aquarium- or boiled pond-water, or with boiled leaves collected from ponds to which boiled aquarium- or pond-water was added. Details of these culture methods were given in an earlier paper (1, p. 402). Occasionally ‘wild ‘Spirostoma, collected from a pond on Coe Fen, Cambridge, or from one in Cheshire, were used. The study was made almost entirely upon the large variety of Spirostomum known as Spirostomum ambiguum Major (21), but Spirostomum ambiguum Minor was studied sufficiently to show that the structures are similar in the two varieties.
A number of different fixatives were tried, including Schau- dinn’s solution with varying amounts of glacial acetic acid below 5 per cent. ; the alcoholic modification of Bonin’s fluid ; strong Flemming’s fluid ; 1 per cent, osmic acid ; the picro- mercuric fixative used by...
