CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 199 



the corresponding pairs of all the other figures also shows a 

 striking correspondence of form in these chromosomes. The 

 homologues of the unusual cells represented by figures 31 and 32 

 may not be correctly determined, as previously mentioned. 



Such agreement and constancy of form between homologues 

 and between corresponding pairs of different individuals cannot 

 well be considered as due to chance and indicates a definite 

 organization of the chromatin. 



3. Summary. The strongest evidence for the existence of pairs 

 is the fact that the chromosomes indicated as pairs 1, 2, and 8 

 in figures 33 and 34 completely satisfy the criteria. Although 

 these pairs fail to do so in figures 35 and 36, they are recognizable. 

 Among those chromosomes composing the two large groups in 

 which the chromosomes differ so little in length (pairs 3 to 7 

 and 9 to 14) the evidence presented by the lengths of the chro- 

 mosomes does not conclusively demonstrate nor deny the exist- 

 ence of pairs because of the various factors inherent in the 

 nature of the material. However, as represented in the graphs 

 and figures, the lengths and forms of these chromosomes strongly 

 indicate such a duplexity. The cases in which the chromosomes 

 of a pair differ somewhat in length do not constitute contrary 

 evidence since homologues are not always of equal length as 

 explained on page 217. Furthermore, the constancy of chromo- 

 some number, the presence in all the cells of certain groups com- 

 posed of the same number of chromosomes with approximately 

 the same relative lengths' is further strong evidence of a con- 

 stancy of chromatin organization and that the lengths of the 

 chromosomes are not due merely to chance. 



It seems to me to be a very difficult task to demonstrate 

 conclusively by means of measurements the existence of pairs 

 in these and similar somatic complexes. To furnish anything 

 more than strong supporting evidence is almost impossible 

 because of the various difficulties inherent in the nature of the 

 material and because of the fact that homologues, as shown in 

 exceptional cases, are not always of equal length, a fact which 

 has been actually observed in the germ cells (tetrads) of certain 

 Orthoptera by several authors. The same conditions make it 



