CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 207 



and again disappeared when thoroughly understood (e.g., Meta- 

 podius Wilson, '10, and the sex group of Ascaris lumbricoides 

 Edwards, '10, and multiple chromosomes of certain Orthoptera 

 Woolsey, '15, Robertson, '16, and McClung, '05, and '17). 2) 

 In considering the significance of variations, it should be remem- 

 bered that there are normal and abnormal conditions (p. 202). 

 3) Metz ('16), Hance ('17 a, b), and AMiiting ('17) have called 

 attention to the necessity of proper technique. This is not 

 always an easy matter to judge, especially in absence of material 

 for comparison. 4) In determining the presence or absence of 

 variation in any material, a very rigid line should be drawn 

 between accurate enumerations and those involving varying 

 degrees of interpretation, e.g., Winiwarter's ('00) cited varia- 

 tions in the amnion and omentum of rabbit embryos were uncer- 

 tain and interpreted. 5) Finally, observers should maintain an 

 exacting standard in distinguishing between that which is con- 

 sidered 'certain' and that which is interpreted. This is 

 especially true in counting small chromosomes. 



E. Fragmentation 



Hance ('17 b, '18 a) found the spermatogonial number in the 

 pig, and the diploid pollen-mother cells in Oenothera scintillans 

 to be constantly forty and fifteen, respectively. But the somatic 

 chromosomes vary from forty to fifty-seven in the pig and from 

 fifteen to twenty-one in Oenothera scintillans. He presented 

 metrical evidence that this variation is due to a fragmentation, 

 probably of the longer chromosomes. He maintains that these 

 fragments divide normally with the other chromosomes, and 

 that therefore this fragmentation does not oppose the theory of 

 the individuality of the chromosomes. 



However, the probability that this variation in Oenothera is 

 much less, and that most if not all of these fragmentations are 

 invisibly connected with the main part of the chromosome is 

 strongly supported by the conditions in Ambystoma material 

 and by Hance's ('18 b) later observations upon additional 

 Oenothera material from the same source. Both he ('17 b, p. 90) 



JOURNAL OF MORPHOLOGY, VOL. 33, NO. 1 



