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SUMMARY. An unsporulated oocyst of Eimeria brunetti encountered at random in a mild infection would probably be 13.8 to 33.7 μ long and 12.4 to 26.2 μ wide. Its shape‐index (length/width) would probably lie between the extremes of 1.0 and 1.6. Its dimensions would probably not change significantly during the sporulation process. If a sample of the oocysts randomly encountered happened to be the progeny of a single inoculation, their mean size would depend, among other things, on the day of the patent period the sample was taken, as well as on certain undetermined factors in the individual host; but their mean length would probably lie between 21.1 μ and 27.1 μ and their mean width between 17.5 μ and 21.7 μ. The mean dimensions would probably be smaller early in the patent period of the infection than later. By a certain process of reasoning it was deduced that the individual components of an “ideal” sample of oocysts of E. brunetti would range from 18.4 μ to 28.4 μ in length and from 15.2 μ to 23.9 μ in width; their mean dimensions with standard deviations would be about 23.4 ± 2.0 μ by 19.7 ± 1.7 μ, and their shape‐indexes would range from 1.0 to 1.4 and average about 1.19. The lengths, widths, and shape‐indexes of the oocysts would usually describe unimodal frequency distribution curves, most of them near‐symmetrical. The mean size of the matured spores produced inside such oocysts would probably be close to 13.2 by 7.5 μ, and their shape‐index near 1.76. As a matter of fact, such a sample would be encountered only by pure chance, because the size of the oocyst of E. brunetti remains an undetermined quantity until it is translated into a veritable definition that takes account not only of its variability under one set of conditions, but also of its capacity to respond to other conditions, among them the size of the inoculation, the influence of the individual host, and the day of the infection. It would be hazardous to discriminate between or among species of coccidia on the basis of oocyst measurements, particularly when they lack conspicuous diagnostic morphological or physiological features of another nature. As Cheissin reflected, observations on a single finding of oocysts, particularly when they are sparse in numbers, are not a substitute for repeated observations on oocysts issuing from the host at different times and over a considerable period.

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SUMMARY. Measurements of the length and width of a total of 6,915 unsporulated oocysts of Eimeria necatrix from 11 infected chickens revealed a length range of 12.1‐28.9 μ, width range of 10.8‐23.8 μ, and a shape‐index (length/width) range of 1.0‐1.64. The average size range of the samples of 25‐100 oocysts (mean, 66 oocysts) constituting this total and taken on the various days of the patent periods was 15.6–23.9 μ× 13.9–19.5 μ. The lengths, widths and shape indexes of the oocysts constituting such samples usually described unimodal, near‐normal frequency distribution curves. The average size of oocysts issuing from an average chicken on an average day of the patent period of the infection was, with standard deviations, 19.7 ± 1.82 μ× 16.7 ± 1.20 μ. The average standard errors for the length and width were 0.237 μ and 0.158 μ, respectively, in samples of the size indicated. A tendency to increase slightly, though significantly, in size after the first three days of the patent period was noted; also, a slight, though significant, linear increase from the first to the seventh day, inclusive. A slight, though significant, tendency to shorten with the passing of the days of the patent period was detected. Analysis of variance revealed much variation in oocyst size from bird to bird on a given day and for given bird and day combinations, but significant day to day variation was demonstrated for width only. By splitting the variance during the first seven days of the patent period into its parts, it was determined that the mean oocyst size during this period was, with standard errors, 19.59 ± 0.238 μ× 16.63 ± 0.200 μ. Further analysis disclosed that a considerable reduction in oocysts measured could be accomplished, with no loss of accuracy, if fewer measurements were taken per bird and more birds were employed.