DIMORPHISM IX SPERMATOZOA OF ANASA. 



297 



With ocular no. 8, seven hundred ten individuals of ir were 

 measured. This number has been shown to be sufficiently large 

 to plot a reliable frequency curve. Spermatozoa selected were 

 taken as random samples from all parts of the slide. The 

 variability curve is shown in Fig. 2. It shows two distinct modes 

 at the lengths 28.08^1 and 30.24^, with a point of depression 

 midway between at 29.16^. Not only are the two modes equally 

 distant from the intermediate low point, but the entire curve is 



Frequency. 



Length in ^. 22.68 23.76 24.84 25.92 27.00 28.08 29.16 30.24 31-32 32.40 33-48 34-56 35.64 



FIG. 10. Bimodal curve of variability of chromatic-rod length for 3r and 3! 

 combined. Number of spermatozoa around upper mode, M", is greater. 



approximately bilaterally symmetrical. This same preparation 

 was measured again with a no. 2 ocular and yet again with a 

 no. 12 ocular. In the use of no. 2 ocular five hundred twenty- 

 four individuals were measured; in the use of no. 12 ocular four 

 hundred twenty-nine measurements were recorded. Both of 

 these give curves agreeing closely to the bimodal curve of the 

 first measurement. 



With ocular no. 2, the magnification was hardly sufficient to 

 correctly place the modes. Ocular no. 8 was found most satis- 



