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measurements in millimetres so obtained by the proper reduction factor found 
by calibrating with a stage micrometer divided to hundredths of a millimetre, 
they were reduced to microns, and so recorded. The arrangement of microscope, 
camera and paper, was of course kept the same throughout the measuring. 
With reference to the selection of individuals to measure, it may be said that 
in Series A, B, C, D and E specimens were taken entirely at random. Every 
pair of conjugants, not obviously distorted, which vvei-e found on a slide were 
measured. When the dimensions of the conjugant pair had been marked off, 
the two undistorted non- conjugants which were nearest to the conjugant pair in 
the field of view were measured. By taking the non-conjugants in this way 
any sub-conscious tendency to choose with reference to size characters was 
avoided. No selection of conjugant pairs on the basis of size was exercised, or was 
even possible, because all conjugant pairs seen were measured with the excep- 
tion of a very few — less than 15 pairs out of all measured — which were obviously 
distorted. In Series AA no non-conjugants were measured. The conjugant pairs 
were taken as in the other series, i.e., all undistorted conjugants on a slide were 
measured. In Series F the conjugants were chosen with reference to nuclear 
condition and separated into two groups on this basis. One group—" Series F, 
Early" — consisted of specimens in veiy early stages of the conjugation process 
and included only individuals in which the micronucleus had not yet divided, 
and either had not moved out of its notch in the macronucleus, or if it had, was 
still lying near and anterior to it. The other group — " Series F, Late " — included 
only pairs in which separation was soon to ensue, where namely the exchange 
of nuclear material was in process or had already taken place. For two reasons 
it was very difficult to get even as many specimens as were obtained in these two 
sharply separated stages. In the first place, in the very early and very late stages 
of the conjugation process the nuclear changes go on rather rapidly, and as 
they are perfectly continuous it is obvious that of any single restricted stage 
there will not be a large number of representatives. Furthermore, sonie were 
rejected which should undoubtedly have been included, but in which, on account 
of the stain having faded, it was impossible to be certain of the stage. No 
non-conjugants were measured in Series F. 
The characters measured were the length and greatest breadth of the body, 
in all the series except F, where only the length was measured. In taking 
these measurements the length offered no difficulty, nor did the breadth in the 
case of the non-conjugants*. In the case of the conjugants, however, the breadth 
* Note added Nov. 10. Mr Lister (loc. cit.) says : " Paramecium, as is well known, is not a 
symmetrical animal. It has been described as 'slipper-shaped' — not a very good comparison, but 
it will serve to bring out the fact that the proportion of length and breadth presented to the observer 
will vary according to the aspect from which the individual is viewed." With all due respect to 
Mr Lister's authority I wish to point out the fact that this statement is entirely erroneous so far as 
the maximum breadth dimension of a normal Paramecium is concerned. The maximum breadth falls 
at a transverse plane of the body posterior to the oral groove. At the level of the maximum breadth 
the outline of a cross-section of the body of Paramecium is circular, and the maximum breadth 
Biometrika v 29 
