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 nearly 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 chropiosome 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 2.5/x (one division of the fine adjustment wheel) 
in any case. Corrections 5 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. 
6 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 (2.5 n 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. 
