CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 217 
than adjacent pairs. The evidence in the other figures is but 
little inferior to that of figures 33 and 34. However, measure¬ 
ments of the chromosomes of so few cells are insufficient to 
furnish more than strongly supporting evidence of the existence 
of pairs, and of an approximate constancy of size relations 
between pairs in different individuals. 
It may appear that these measurements support equally well 
Della Valle’s claim that there are no chromosome pairs, but that 
the chromosomes form a series of variants. However, the con¬ 
sistent evidence of the presence of pairs among the shorter 
chromosomes, the possibility of unequal stretching of the longer 
homologues together with the known condition in Orthoptera 
that homologues of tetrads may be of unequal lengths lends 
greater support to the probability of the existence of homologues. 
The following points need further consideration. 
Large differences in length between homologues . . The difference 
in length of 12.4 mm. between the homologues of pair 9 in 
figure 33 and the similar difference of 9.5 mm. between the 
homologues of pair 12 in figure 37, the smaller difference in 
pairs 7 and 8, figure 35, pair 7, figure 36, and pair 8, figure 37, 
may possibly be explained as follows: 
1. Unequal homologues have been reported in Orthoptera by 
Baumgartner (’ll), Hartmann (’13), and more thoroughly studied 
by Carothers (’13), Robertson (’15), and Wenrich (’16). The 
latter’s observations are particularly significant. He found dif¬ 
ferent conditions of inequality in two of the small tetrads of 
Phrynotettix. He designated these two tetrads as ‘B’ and 
‘C’ and traced their history from the pachytene stages through 
the first maturation division. The homologues of tetrad ‘B’ 
were unequal in eleven of the thirteen individuals studied and 
were equal in the other two. Tetrad ‘C’ was found in three 
forms, designated as ‘Ci, C 2 , C 3 .’ ‘Ci is composed of very 
unequal elements, the larger of which possesses a relatively large 
terminal knob or granule which is not present on the other two. 
‘C 2 ’ is a pair with equal members, each of which appears to be 
homologous to the smaller member of ‘Ci.’ ‘C 3 ’ is a pair of 
unequal elements, neither member of which appears to be 
