CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 187 



described above and have to do only with relative lengths. This 

 fact eUminates at once a number of errors which would otherwise 

 be very serious. 



The instrumental errors. The possible instrumental errors are 

 a) failure to maintain a critical illumination, b) failure to maintain 

 a constant wave length of illumination, and c) errors inherent in 

 the planimeter and opisometer. 



It so happened that the strongest light was obtained a little 

 above the point of critical illumination, and might therefore 

 cause an error in measurement. However, several chromosomes 

 were drawn a number of times under both conditions and no 

 perceptible difference was observed. Any slight error overlooked 

 would be equal in the homologous chromosomes and would not 

 interfere with a relative measurement. 



Farmer and Digby ('14) state that errors can arise from the 

 use of varying wave lengths of light. The same optical equip- 

 ment and illumination were maintained in all of my operations 

 so that relative values were unaffected. 



In making measurements with the planimeter the polar arm 

 was held rigidly stationary in two grooved blocks so that the 

 tracing point moved around in a circle having a diameter of 33 

 cm. The sharp tracing point was kept upon the median line of 

 the chromosome figure by moving the drawing around (without 

 slipping) into line with the path of this tracing point, which was 

 used as a pivot for orienting the drawing. A constant, repre- 

 senting the value of each of the divisions of the vernier, was 

 determined by measuring a series of known distances on a straight 

 line. 



The accuracy with which this instrument was operated is 

 indicated by the fact that the average difference between the 

 extremes of measurements made upon each of several drawings 

 is 0.3 mm., the standard deviation, computed from the combined 

 measurements of several drawings, is 0.17 mm. The measure- 

 ments, obtained more quickly with the opisometer, are slightly 

 less accurate, the average of the above extremes and the standard 

 deviations being 0.4 mm. and 0.26 mm., respectively. Finally, 

 the average of the nine measurements made upon the three 

 drawings of each chromosome reduces this instrumental error 



