CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 219 
cells prevents extensive investigation of this point in the somatic 
cells for the present and makes this explanation only suggestive. 
Constancy in the individual. It should be emphasized that the 
observations in the Orthoptera concerning the unequal tetrads 
(p. 217) and other heteromorphic tetrads (p. 192) strikingly 
demonstrate a constancy in the individual for the particular 
characteristic of each homologue concerned. In Ambystoma it 
has been impossible to obtain sufficient material to verify this 
point. 
Whether the members of these Orthopteran unequal and other 
heteromorphic tetrads maintain their organization from one 
generation of animals to the next is yet to be demonstrated by 
breeding experiments now in progress in this laboratory. The 
expectation is that they do, since Wenrich (T6) and Carothers 
(’17) find every possible combination which would arise from the 
segregation and recombination of the members of these various 
types of tetrads. 
The presence in the Orthoptera of unequal tetrads does not 
indicate a lack of individuality. On the contrary, the per¬ 
sistence of this condition throughout the individual, and perhaps 
from generation to generation, is strong evidence to the con¬ 
trary. Of course a change has taken place at some time (if it 
be correct to assume that the homologues were all alike at some 
earlier period), but this is to be expected if these chromosomes 
are to parallel genetic behavior. 
Bridges (’17, p. 445-6) presents parallel genetical data in 
connection with the chromosomes of Drosophila. He finds in 
certain cases that the genes for ‘bar’ eye and ‘forked’ bristles, 
whose loci are located near one end of the sex-chromosome, have 
been lost and that the region between these two loci has also 
been affected. He suggests that this deficiency may be due to 
a physical loss of this portion of the chromosome. He also 
reports (T9, p. 357) a case in which “a section of the X-chromo- 
some, including the loci for vermilion and sable, became 
detached from its normal location in the middle of the X-chro- 
mosome and became joined on to the ‘zero’ end (spindle fiber) 
of its mate.” In other instances the locus for sable alone, as 
