CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 217 



than adjacent pairs. The evidence in the othei' figures is but 

 Httle 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 

 Delia 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 possiblj^ 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 then- 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, C2, C3.' '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. 

 ' C2' is a pair with equal members, each of which appears to be 

 homologous to the smaller member of 'Ci.' 'C3' is a pair of 

 unequal elements, neither member of which appears to be 



