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 Whiting (’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 
