CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 1.95 
homologues of the pairs cannot be determined with certainty, 
but they doubtless exist. Since the measurements of figures 33 
and 34 are very nearly accurate and present the most reliable 
evidence, these will be considered separately from the others. 
For convenience the pairs may be considered in two groups, the 
first including those which differ greatly in length from their 
neighbors (pairs 1, 2, and 8) and the second including the 
remainder in which the pairs are not so clearly distinguished. 
It is all but certain that the chromosomes represented in each 
of pairs 1, 2, and 8 in both of these figures are homologues, for 
they almost completely satisfy the criteria outlined above. In 
these pairs there is foreshortening in only one chromosome and 
in pair 8 the error of 0.8 mm. in both complexes due to twisting 
of the chromatids is negligible. The homologues are of approxi¬ 
mately equal length, and the difference between each pair and 
the adjacent pairs is so much greater than the error of measure¬ 
ment that it is improbable that the condition represented by 
these three pairs in both complexes is merely a matter of chance. 
Furthermore, there is a close resemblance of form between these 
homologues. A comparison of other cells of the same individual, 
if available, would be expected to show that this condition is 
constant in all the cells as is shown by comparable cases of 
constancy in the germ cells of individuals of certain Orthoptera 
(p. 219). It can, therefore, be maintained with considerable 
confidence that these particular chromosomes of equal length 
and sameness of form actually constitute pairs. The measure¬ 
ments of the chromosomes of these pairs in other cells as dis¬ 
cussed below give similar although less conclusive evidence. 
Among the chromosomes of the second group in these two cells 
there is strong evidence for the existence of pairs, but the small 
difference in length and the errors due to twisting and possible 
stretching make it inconclusive. In figure 33 the homologues as 
shown in each of pairs 3 to 7 are so nearly of the same length 
and form (fig. 27) that one may believe that they constitute 
pairs as represented. Pair 3, in addition, is well separated from 
those adjacent. Although pairs 4 to 7 appear to be actual 
pairs, the chromosomes of this series differ so little in length that 
