Summary. Experiments are described on carbon dioxide production from soil and sand cultures containing a species of bacterium with and without amoebae. The following results were obtained: Carbon dioxide production and bacterial numbers are correlated provided that amoebae are not present, or are present in very small numbers. The bacteria are more efficient as producers of carbon dioxide when their numbers are not rising and less efficient when their numbers are increasing. This does not hold for young cultures. Also each bacterium becomes less efficient as the density of the population increases. The amoebae cause a decrease in carbon dioxide production in sands containing peptone, but an increase in sands containing mineral salt solution with glucose or soil extract

Living bacteria form the usual food of the small soil amoebae, but numerous references in the literature show that different bacteria have different effects on the amoebae (

THE rate of growth and the number of divisions a unicellular organism undergoes in a given period of time has attracted the attention of biologists for many years. Numerous publications by bacteriologists have appeared on this subject; and the work of Maupas [1888], followed later by. that of Woodruff, Calkins and others, has brought it into prominence as regards protozoa.In 1923 we published the results of certain experiments on the ciliate Colpidium colpoda, our object being to show the necessity of keeping -experimental conditions rigidly constant if comparable results were to be obtained. Among other variable factors the food supply was considered. It was realised that this was a factor particularly difficult to control, owing to the fact that the culture of the ciliate was contaminated by a species of a small bacillus. This was extremely difficult to exclude from the culture; and, moreover, when its numbers were reduced the ciliates showed typical signs of hunger degeneration, makilg it impossible to carry sub-culturing further. To obviate the difficulty, therefore, we decided to give to each sub-culture, either at the time of inoculation, or after 24 hours' growth, a plentiful supply of an easily recognisable species of bacterium, Sarcina lutea, in the belief that so long as plenty of these were present the food supply would be more than adequate to meet the requirements of the protozoa. Oiur experimental results appeared to justify this assumption, which also received support from the work of other observers, who had worked on a similar basis. Further, as the ciliate we were using was of the same strain as that used by Peters [1921], for his work on the growth of protozoa without bacteria, the possibility of deficient bacterial food supply seemed remote.Recently, however, we have had to revise our views for the following reasons. Experiments were started to test which of the two bacteria, the contaminating bacillus or the added Sarcina, provided the better food supply. This involved counting the numbers of bacteria and protozoa in the cultures; whereupon it at once became evident that there was a very definite relationship between the number of divisions of the ciliate and the numbers of bacteria present.

IN a previous paper [Cutler and Crump, 1923] an account of experiments on mass cultures of Colpidium colpoda was given; the present communication deals with similar experiments carried out on cultures containing one or more cells isolated into one cubic centimetre or less of culture solution. The objects of these experiments have been to discover whether the results obtained from cultures containing few organisms in small quantities of fluid are comparable with those in which numerous animals are inoculated into comparatively large amounts of liquid, 10 cc. or more. Also there have appeared a series of papers by Robertson [1921Robertson [ , 1922, describing new and interesting phenomena -allelocatalysis, etc.-in cultures of a ciliate Enchelys farcinem. These seemed to be of so fundamental a character that it was felt desirable to test whether they obtained with other species of ciliates. METHODS.The medium used has been the synthetic one in which the experiments on mass cultures were conducted. The smaller cultures (i.e. less than 0 09 cc. in volume) were put up as follows in unruled counting chambers 041 mm. deep: the individual, or individuals, are transferred, by a capillary tube from the parent culture, with about 0 01 mm.3 of liquid, to the chamber. It has not, however, been possible to keep this quantity rigidly constant, owing to the small volume of the fluid used; the chamber is then covered with a thick cover slip, on which there is already a drop of new medium, freshly inoculated with Sarcina to provide an adequate food supply, and the two drops are allowed to mix. The final size of the drop mayvary from 0 37 to 9 8 mm.3, and the degree of dilution varies between 1 in 37 and 1 in 980; these dilution figures, however, are only an approximation owing to the difficulty of measuring the original minute drops containing the organisms with accuracy, and, for the same reason, the size of the drop is not of necessity an index of the degree of dilution. Before the cover slip is put on the chamber a ring of small drops of sterile medium are placed in the groove around the raised central part to discourage

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