CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 221 
measured. This confirms the contention of a constancy of 
chromatin organization so far as is possible in material having 
chromosomes differing so little in length as those of Salamandra 
maculosa. 4) The measurements of chromosomes in the so¬ 
matic cells of Ambystoma tigrinum show duplication of sizes, 
especially pairs 1, 2, and 8 in figures 33 and 34, pairs 8, 9, and 10 
in figure 36 and indications of the same in the other chromo¬ 
somes of all the cells which differ too little in length to constitute 
reliable evidence. Explanations are offered for cases in which 
the homologues differ in length. 5) The chromosome lengths 
show approximately constant relative sizes in all of the cells 
measured. 
Based upon the above considerations and upon the unequal 
and other heteromorphic tetrads in Orthoptera, my expectations 
are that the pairs and their relative lengths in the somatic cells 
of Ambystoma are constant for the individual, and although 
not exactly jthe same, they are approximately the same in dif¬ 
ferent individuals. However, as stated above (p. 199), the meas¬ 
urements cannot be considered to demonstrate conclusively the 
presence of a duplicate series of chromosomes. 
SUMMARY OF CONCLUSIONS 
1. No variation is found in the somatic chromosome number 
of twenty-eight in Ambystoma tigrinum. 
2. Della Valle’s contention that variation in chromosome 
number is the rule is unconfirmed. 
3. The chromosomes form approximately a duplicate series of 
sizes and forms, supporting the contention that they consist of 
pairs of maternal and paternal homologues. 
4. An approximate constancy of size relations between pairs 
in the complexes of different individuals is also maintained. 
5. Della Valle’s claim that the chromosome lengths are a series 
of variants is not substantiated. 
6. There is evidence of unequal homologues in these cells. 
7. There is also a suggestion that there is a sex chromosome 
attached to a euchromosome. 
