186 CHARLES L. PARMENTER 



under which these measurements were made in order to judge 

 their value correctly. 



1. Type of cells. Only cells were used in w^hich every chro- 

 mosome was perfectly clear and, except as noted (p. 189), lay 

 exactly level in the equatorial plane throughout their entire 

 length. Only three cells (figs. 1, 3, and 9) of this quality 

 were available, and these were polar views of early metaphase 

 stages in cells of the peritoneum and lung. The chromosomes 

 of one other cell (fig. 10) approximated this condition and were 

 also measured. The care with which these cells have been 

 chosen may be judged from the fact that they were the only 

 suitable cells in material from over one hundred larvae con- 

 taining large numbers of division figures. In material with chro- 

 mosomes so long and so numerous it is not surprising that so few 

 cells were perfect enough for measurement. 



2. Method. In addition to choosing cells with chromosomes 

 of the above character, three different camera-lucida sketches of 

 each chromosome w^ere made on different days with extreme care 

 at a magnification of 2633 diameters. Each of these sketches 

 was measured three or more times along the median line with an 

 Ott compensating planimeter modified for this purpose, or with 

 an opisometer. These nine determinations obtained for each 

 chromosome were averaged to represent its length. This method 

 is important because the extremes of these nine measurements in 

 about one-fifth of the cases may differ 1 mm. from the average 

 (and occasionally more). This demonstrates that one measure- 

 ment upon a single drawing might give rise to an erroneous 

 difference in the lengths of the homologues of some pairs rang- 

 ing from 1 to 2 mm., the actual amount depending upon the 

 respective errors in each homologue. Averages largely ehminate 

 this error. 



3. Sources of error. The various sources of error may be classi- 

 fied in three groups : 1) instrumental errors, 2) personal errors, 3) 

 errors inherent in the condition of the material. 



In the first place, it should be emphasized that no attempt 

 has been made to determine the actual length of any chromo- 

 some. These measurements have all been made on the drawings 



