1921] CURRENT LITERATURE 181 
and FARNHAM? have discovered this phenomenon in Datura. The normal 
diploid number of chromosomes in this form is twenty-four. Twelve different 
“mutants” have been discovered with twenty-five chromosomes. This seems 
to indicate that each of the twelve chromosomes (haploid) has failed to disjoin 
at least once in history. These twelve new forms are abnormal in their vege- 
tative features, and notably low in fertility. 
ETRAPLOIDY.—A hurried or incomplete mitosis will sometimes result 
in te simultaneous duplication of all of the chromosomes. This phenomenon 
has been observed several times, and there are indications that it has taken 
place frequently in the past. A general survey of the chromosome counts 
emphasizes the fact that the haploid number is much more frequently an even 
number than an odd one. This, together with the fact that there are several 
species groups in which the chromosome count of some of the members is just 
twice that of the others, suggests that tetraploidy may have played a con- 
siderable réle in evolution. Tetraploidy commonly, but not always, brings 
gigantism. 
BLAKESLEE now puts the finishing touches on this tetraploidy conception 
by more work on Datura. In addition to the abnormal forms with twenty-five 
chromosomes, he has discovered one completely triploid (thirty-six) and 
one tetraploid form. These latter both seem to be in a> “better balanced” 
condition than the non-disjunctional (twenty-five) forms, since they are more 
“normal” with respect to their vegetative features and fertility. The beauty 
of the situation arises from the fact that the tetraploid type contains a pre- 
viously known Mendelian factor. In normal forms a hybrid of the compo- 
sition Aa will give a 3:1 ratio of purple- and white-flowered in the F,. The 
tetraploid form AAaa gives gametes in the ratio 14 A:4Aaz1aa. These 
recombine to produce an F; of 35 purple:1 white. The F; and later generations 
behave according to expectations on this basis. 
A question of terminology now arises. Of these various.types of germinal 
changes, it seems the consensus of opinion to restrict the term mutation to the 
locus change. This is undoubtedly the most frequent type of change to = 
place, and possibly the most effective single factor in evolution. Deficienc 
and duplications are very rare at best. Non-disjunction and Siacr 5 are 
probably fairly common, and the latter is doubtless very important in evolu- 
tion. These last two (and panes Laces as well) may be referred to 
collectively as “chromosome aberra 
this differs from maging as originally described by DE VRIEs. 
This is not surprising in view of the fact that the original example of muta- 
tion was not a true case of mutation at all; it now seems certain that O. Lamarck- 
iana is a hybrid, and its “mutants” merely recessives being segregated out. 
? Braxestits, ALBERT F., BELLING, JOmN, a0 and Farnuam, M. E., Chromosoma 
duplicati ‘ ts. Science 52:388-390. 1920. 
