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 ('16) 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 ('19, 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 



