52 Genetical and Cytological Studies on Pisum 



change does not result in ring formation, but the gametes with one part 

 reduphcated and deficient in the other are abortive, there should always 

 be 50 per cent, gamete abortion, since the segregation of the chromosomes 

 is at random; but where a ring or chain is formed there is a possibDity 

 of controlled segregation with a high proportion of disjunction, as is 

 apparently the case in the cross between Line 1 and "B" whites. 

 Although the suggestion that semi-steriUty was caused by ring formation 

 was made as a result of the work on Datura, no clear case of semi-sterility 

 associated with ring formation has been reported in that genus. 



In the present state of knowledge it is impossible to say why a ring 

 should sometimes give disjunction and non-disjimction in equal pro- 

 portions followed by an abortion of 50 per cent, of the pollen grains, as 

 in Pisum and Zea Mays, whereas in Datura a ring has been observed which 

 apparently gives a high percentage of disjunction and is followed by no 

 considerable pollen abortion. There are two factors which may con- 

 ceivably afEect disjunction in the ring. One is the position of the attach- 

 ment constrictions of the chromosomes with reference to the interchanged 

 segments. If there is a sufficient distance between the point of inter- 

 change and the attachment constriction there is the possibility of chiasma 

 formation between two homologous segments on both sides of the 

 attachment constriction, a phenomenon which would give rise to special 

 configurations. Such configurations have not as yet been identified in 

 the Pisum material studied, but it is possible that they may occur 

 occasionally. 



The second factor which may afiect disjunction is the extent of 

 terminahsation of the chiasmata. In Pisum and Zea Mays, ter- 

 minaUsation is slow and interstitial chiasmata are present at meta- 

 phase — whereas in Datura terminalisation is usually complete. Complete 

 terminahsation also occurs normally in Oenothera, where the extent of 

 non-disjunction is not so great as in Pisum and Zea Mays. 



Closer investigation of these and other cases of ring formation will no 

 doubt throw Ught on the causes affecting disjunction and non-disjunction. 



Summary. 



The fourteen chromosomes of normal races of Pisum sativum form 

 seven pairs at meiosis. All points of association between chromosomes 

 are shown to be chiasmata by the following considerations : 



(1) Direct observation of exchange of partners amongst the chro- 

 matids. 



(2) Alternate loops between points of contact are at right angles. 



