CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 193 



rates, and that unequal stretching, especially between the 

 longer chromosomes, may occur during mitosis, increases the 

 difficulty in obtaining accurate metric comparisons, and may 

 interfere with perfect certainty in the interpretation of the 

 results. Furthermore, some variation is characteristic of li\dng 

 material and hence sHght differences in relative length and form 

 in different cells would be expected rather than absolute uni- 

 formity. McClung ('17, p. 567) finds in certain Orthoptera 

 that the accessory chromosome, although unmistakably dis- 

 tinguished from the euchromo somes, is not always of the same 

 relative length in different individuals of a given species, for it 

 sometimes occupies the fourth and sometimes the fifth position 

 in the series of lengths. However, it must be remembered that 

 this is the sex chromosome which at the metaphase (the stage 

 of greatest condensation of the euchromosomes) is already be- 

 coming diffuse. Differences in the degree of condensation might 

 therefore be involved in the differences of relative lengths. And 

 again, since individuals vary in their morphological character- 

 istics, why should it be expected that the chromosomes of two 

 different individuals should be of exactly the same relative 

 lengths at the same stage of mitosis? In view of universal 

 variability, homologous chromosomes, which are derived from 

 different individuals and which may be expected to maintain 

 their individuality, should not invariably be of exactly the 

 same length. As discussed (p. 217), the observations on different 

 Orthoptera by several authors indicates this to be so. 



On account of these interfering factors it cannot be expected 

 that homologous chromosomes will always be of exactly the 

 same length at any particular stage of mitosis. Therefore, 

 length and form, considered in a limited number of cells, from 

 different individuals, cannot be regarded as conclusive evidence 

 for or against the presence of chromosome pairs. Much more 

 conclusive evidence would be had in a comparison of several 

 somatic complexes from a single individual and with those of 

 other individuals, a comparison of these with the diploid and 

 haploid chromosomes of the germinal line, and a comparison of 

 the complexes of parents and progeny. 



