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 living 
material and hence slight 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 euchromosomes, 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. 
