NON-DIVISION OF CENTROMERE 427 



the anaphase chromatids. This shows that the division of the 

 chromosome cannot have been aheady determined at the preceding 

 meiosis and, further, that the division of a chromosome may be 

 conditioned by its having a centromere at least at the beginning of 

 the resting stage (Plate IX). 



When structural changes occur between chromatids it is possible 

 to define their character in a way that is not possible with chromo- 

 some changes, for the chromatids retain the association with their 

 homologous partners and hence give pseudo-chiasmata. These 

 chiasmata in some cases clearly result from asymmetrical inter- 

 change and show that this is how dicentric and acentric chromo- 

 somes arise when they are produced by change in the undivided 

 chromosomes. Pairs of chromosomes with these chiasmata behave 

 normally at metaphase and do not orientate themselves hke meiotic 

 bivalents (White, 1935). 



Treatment has no effect on metaphase or anaphase chromosomes 

 until half an hour has elapsed, but earlier stages of mitosis (Strange- 

 ways and Hopwood, 1927 ; Stone, 1933) are arrested for twenty- 

 four hours or more in root-tips. This delay no doubt results from 

 general damage to the cell such as is shown by the plastids (Hruby, 

 1935)- Where the effects are more drastic they are also more 

 complicated. White (1935) has distinguished three kinds of change, 

 the probable causation of which may be inferred, in spermatogonia 

 of Locusta. (i) A proportion of nuclei are killed, whether at a 

 particular stage of mitosis or not cannot be said, (ii) All or many 

 cells treated in the early prophase are delayed in coming to meta- 

 phase so that mitoses with chromatid breaks do not appear until 

 twenty hours after the first mitoses with chromosome breaks. 

 This suggests that during the prophase the normal development of 

 the centrosomes or spindle-determinants is upset by the cytoplasmic 

 abnormality resulting from treatment, (iii) Occasionally nuclei 

 appear, sometimes in groups, whose chromosomes have an anomalous 

 structure. They have four chromatids instead of two attached to 

 each undivided centromere. These diplochromosomes differ from 

 the attached X-chromosomes in Drosophila, in that the homologous 

 arms are attached to the same side of the centromere and not to 

 opposite sides. The origin of the diplochromosomes can be 



