CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 191 
Summary. The combined instrumental and personal errors 
are reduced to practically zero by averaging several measurements 
made upon different drawings. The errors due to twisting of 
chromatids about one another are largely neutralized and are 
considered individually later. Therefore, except for possible 
unequal contraction and stretching of homologues, only the 
measurements of those chromosomes which are foreshortened 
contain appreciable errors. It is thought that these errors, 
after corrections have been made, probably do not in any case 
exceed 1 mm. The presence and amount of error due to unequal 
contraction and stretching in any particular chromosome is an 
uncertainty, but if existing would probably be greater in the 
longer chromosomes. 
c. Results of measurements. 1. Criteria for determining pairs. 
Before considering the results of the measurements, it seems 
desirable to state what the criteria are that will demonstrate 
whether pairs (p. 185) are present among the chromosomes at 
these particular stages of mitosis. In the absence of definite 
minute morphological characteristics, such as a repeated occur¬ 
rence of marked granules, constant in position and size, which 
Wenrich (’16) describes for certain Orthopteran chromosomes, 
the next most exact criterion for determining the presence of 
pairs in diploid cells would be a duplicate series of chromosomes 
of equal volume. But in these chromosomes trustworthy volu¬ 
metric determinations cannot be obtained, for the above- 
mentioned intertwining of the chromatids and the stretching of 
the chromosomes would cause variations in diameter which 
could not be measured accurately, and these errors would be 
cubed in the volume. Consequently linear measurements, sup¬ 
ported by form, have been chosen as giving more trustworthy 
data. 
Upon this basis, in order to constitute undeniable evidence 
that the chromosomes form a duplex series, there are two con¬ 
ditions which should be met. First, when the chromosome 
lengths are plotted in a graph (e.g., figs. 33 to 37), they should 
definitely associate themselves in twos of equal lengths. Second, 
the differences in length between successive pairs, as indicated 
JOURNAL, OF MORPHOLOGY, VOL. 33, NO. 1 
