CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 189 



nification of the drawings (2633 diameters), amounts very 

 closely to 0.4 mm. However, the effect of this condition is 

 either completely or largely neutralized by equal or nearlj- equal 

 amounts of twisting in the homologues of each pair. The maxi- 

 mum amount of error due to this cause may be judged by an 

 examination of figure 28 which contains the most twisting. In 

 this cell there are seven pairs in which the amount of twisting 

 is equal and the error completely neutralized, two pairs con- 

 taining an error of 0.4 mm., two pairs with 0.8 mm. error, one 

 with 1.2 mm., and two pairs in which it is uncertain. Since 

 these errors that occur at critical points will be considered indi- 

 vidually later, no corrections for them are included in the 

 measurements. 



Foreshortening occurs only in certain chromosomes as indi- 

 cated in figures 27 to 30 and 33 to 36. This is, however, in the 

 cells represented by figures 27, 28, 33, and 34 so slight that the 

 whole chromosome can be seen at one focus, the foreshortened 

 part appearing only slightly hazy. In the cells represented by 

 figures 29, 30, 35, and 36 it is a little more. Measurements 

 with the fine-adjustment graduated wheel, made more accurate 

 with a sharper pointer made of a pin, indicated this sagging to 

 be not more than 2.5m (one division of the fine adjustment wheel) 

 in any case. Corrections'^ made for this foreshortening are 



of the shortened portion. Double the length of this hypotenuse minus the orignal 

 measurement of the shortened portion is 0.2 mm., which is the maximum amount 

 of shortening due to the vertical sag of any twisted portion. This amount added 

 to that caused by the lateral deviation made the total shortening in one twist 

 amount to 0.4 mm. Although this determination cannot be considered entirely 

 accurate, it is a close approximation. 



5 The correction was made as follows. The amount of vertical deviation was 

 read from the fine-adjustment wheel when the objective was focused as nearly 

 as could be judged upon the middle of the lowest and highest points of the fore- 

 shortened portions. All measurements for a given complex were made with the 

 same part of the fine-adjustment screw, thus avoiding different pitches in the 

 thread. The reading (2.5m for each division) gave the actual differences of 

 vertical positions. This multiplied by the magnification and divided by 1000 

 converted the figure into millimeters, the units made use of in the drawings. 

 By using this distance as the altitude of a right triangle and the measured longi- 

 tudinal extent of the foreshortened portion as the base of the triangle, the hypot- 

 enuse (which represented approximately the correct length of the foreshortened 

 part) was determined. This was substituted for the original measurement of 

 the foreshortened portion. 



