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THE AMERICAN NATURALIST [Vol. XLIX 



represented can not be made for the filaments are alike in 

 every characteristic. Of the 500 zygospores observed 45 

 per cent, were produced by lateral conjugation. 



4. Methods 



In considering the variability of large numbers of 

 microscopic forms, rapid and accurate measurements are 

 a necessity. Pearl and Dunbar ('03) in measuring Ar- 

 cella used a camera lucida, marking the dimensions by 

 means of a needle point, and reducing to microns. Pearl 

 ('06) adopted a similar method for Chilomonas, using a 

 magnification of 689.7. Pearl ('07) in measuring Para- 

 mecium used a 2/3-inch objective and a No. 1 ocular. By 

 means of a camera lucida the points to be measured were 

 projected on cards, marked, and measured with a vernier 

 calipers to tenths of millimeters. Multiplying the meas- 

 urements so obtained by the proper reduction factor found 

 by calibrating with a stage micrometer, they were re- 

 duced to microns and recorded. Jennings ('11) at first 

 measured Paramecium from a slide with an ocular mi- 

 crometer. Later an Edinger drawing and projection ap- 

 paratus was used, the projected images of the specimens 

 on a slide in a flat drop of 25 per cent, glycerine, with- 



