No. 587] VARIABILITY AND AMPHIMIXIS 657 



out a cover glass which by pressure would have caused 

 distortion, were enlarged to 500 diameters and measured 

 with a milimeter ruler. 



In the present study, the material was preserved in 2 

 per cent, formalin, the first series of measurements 1 being 

 made April 2, while measurements of 358 were completed 

 before May 16, and the remaining 42 finished Aug. 21 of 

 the same year. Swelling of the zygospores did not occur 

 to an appreciable extent, a possible error considered 

 in a subsequent part of the paper. Using a B. and L. 

 BB-6 microscope with a No. 1 ocular and a 1/12 oil immer- 

 sion, a slide with a couple of drops of fluid containing the 

 material was covered with a No. 2 coverglass, the super- 

 fluous liquid drawn off by means of a pipette, and the 

 preparation placed on the mechanical stage. Beginning 

 at the lower right-hand corner the slide was moved from 

 left to right and each zygospore presented in the field in 

 a uniformly horizontal condition, was measured. On 

 reaching the left margin of the preparation, the slide was 

 returned to the first position, moved sufficiently toward 

 the observer so that a new path would be traversed, and 

 the operation repeated. Thus the selection was at random 

 and no zygospore measured twice. The dimensions were 

 marked on note paper by means of a camera lucida at a 

 magnification of 1,460 diameters, the two cross lines 

 representing length (x) and diameter (y) having at the 

 point of juncture an "S" or an "L" for scalariform or 

 lateral conjugation. Only those zygospores having defi- 

 nitely formed membranes were considered. 



In the reduction of data, so soon as the projections of 

 the apparent dimensions were completed, the length of the 

 lines x and y were measured with proportional dividers 

 (Keuffel and Esser No. 441 special) adjusted at the ratio 

 1,460 to 1,000, thus giving a reading in tenths of microns. 

 Accurate adjustment was made possible by means of a 



