48 



Genetical and Cytological Studies on Pisum 



such a region. However, on the hypothesis of terminalisation, crossing- 

 over may have taken place at any point between the attachment con- 

 striction and the metaphase chiasma, since the tendency is for chiasmata 

 to move away from the attachment constriction. The regular stoppage 

 of the chiasma at a particular point may be due to a block at that point. 

 Meurman (1929) found that such a stoppage was caused in Aucuba 

 japonica by the presence of secondary constrictions, and Darlington 

 (Tradescantia, 1929 6) put forward the suggestion that the presence of 



Fig. 9. Configuration caused by the interlocking of two pairs of segments of the ring. 

 From the semi-sterile plant 256V29. The letters in the diagram represent the four 

 pairs of homologous segments, x 7500. 



small non-homologous segments of chromosome might offer a check to 

 terminalisation. This he has now rendered very probable by observations 

 on Oenothera (1931). 



It is hoped that a study of the somatic chromosomes of fertile and 

 "ring-forming" plants from a semi-sterile parent will show whether 

 secondary constrictions are present in the chromosomes concerned with 

 ring formation, or not. 



Fig. 9 shows a special configuration caused by the accidental inter- 

 locking of the ends of two pairs of segments at an earlier stage. The 



