PLATE XII 



The Formation of Acentric and Dicentric Chromatids and 

 Chromosomes by Crossing-over (figs, i and 2) and Irradia- 

 tion (figs. 3-8) 



Figs, i and 2.— Criss-cross separation of dicentric chromosomes in a 

 binucleate pollen grain of a triploid Tulipa following non-disjunction of 

 dicentric chromatids at meiosis. X 1000. (Upcott, unpubhshed.) 



jTjQ 3 — Metaphase in root-tip of Crocus Olivieri. Six centric 

 constrictions, two in one chromosome at 5 o'clock owing to chromosome 

 breakage. (Mather and Stone, I933-) 



Fig. 4. Pollen grain of Allium Moly with pseudo-chiasma due to 



chromatid breakage which follows irradiation at the end of the resting 

 stage. (Mather and Faberge, unpublished. 2B.E. - gentian-violet 

 smear.) 



Fjg 3. — Tulipa root-tip wdth dicentric chromosomes. (Mather 

 and Stone, I933-) 



Fig. 6. — Anaphase of mitosis in a root-tip of Tulipa, showing criss- 

 cross separation of dicentric chromatids. (Mather and Stone, I933-) 



Figs. 7 and 8. — The same in Vicia Faba, showing ring chromatid and 

 ;Vf — interlocking of dicentric chromatids. (Roller, I934-) 



