CHROMOSO:\IE NUMBER AND PAIRS IN AMBYSTO:\IA 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 abo^^e 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 the 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. Delia 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. Delia 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. 



