1922] McDonald: On Balantidium coli and Balantidium sims 251 



iuto a large quantity of it so that action would be as nearly instan- 

 taneous as possible, thus avoiding distortion. Osmic acid vapor was 

 tried but SehaudLnn's fluid gave equally good results and was more 

 convenient for manipulation. Material was never allowed to cool 

 before fixing, for on cooling the individuals tend to become spherical. 

 Several attempts were made to measure living organisms but their 

 ceaseless activity at normal temperature (37.5° C) made this almost 

 impossible and slowing them up by the use of Irish moss or by cooling, 

 as mentioned above, caiLsed them to become distorted. If there were 

 changes due to fixation, the logical expectation would be that the error 

 would be on the side of conservatism for such changes would tend to 

 obliterate rather than accentuate the division into two groups; for the 

 shape of' the elongate forms would be more changed by the fixative, 

 the tendency being for them to shorten and broaden and thus approach 

 the ovoid tj-pe. However, in the method of fixation used, I am sure 

 that distortion was so slight as to be negligible. 



Following fixation the material was carefully washed and carried 

 slowly through the lower grades of alcohol to 70 per cent in which 

 the material was kept for measuring. A drop of the material from 

 which measurements were to be taken was placed on a slide, covered 

 with a coverglass, the excess of fluid removed, and the edges sealed 

 with vaseline to prevent evaporation. Just enough fluid was removed 

 from under the coverglass to reduce the depth of the medium so that 

 the majority of the animals would lie flat, and yet not enough to allow 

 the coverglass to exert any pressure. The exertion of pressure on the 

 animals, however, would ordinarilj- be prevented by the presence of 

 large particles of foreign material. The object of having animals lie 

 flat on the slide was to avoid the error which would otherwise be caused 

 by foreshortening. A slight elevation of one end would make con- 

 siderable error in the determination of the length of the animal. 



The slide was then placed on the microscope and systematically 

 examined by the use of the mechanical stage. Beginning at the upper 

 left-hand corner and progressing as one would in reading a book, eveiy 

 individual encountered in the survey was measured. The only excep- 

 tions made were in case the animal was not lying flat or showed marked 

 signs of distortion. This procedure avoided selection which might 

 unconsciously be made by the obsei-ver. For making the measurements 

 a 4 mm. objective was iLsed in combination with an ocular-micrometer 

 inserted in 9x compensating ocular. "With the magnification given by 

 this combination the limit of error did not exceed one micron. 



