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 paii;s 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 
