CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 195 



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 



