Caroline Pellew and Eva Richardson Sansome 51 



in Zea Mays, where three different lines have been isolated which on 

 crossing with the normal give about 50 per cent, gametic sterility. 

 Genetical and cytological work indicates that difierent chromosomes are 

 involved in the ring in the cases of semi-sterile 1 and semi-sterile 2, so 

 that they give 2 rings of 4 when crossed, whereas semi-sterile 1 and 

 semi-sterile 3 have 2 pairs of segments in common in the ring (so that 

 when they are crossed a ring of 6 occurs). It is interesting to notice that 

 in the cross between semi-steriles 3 and 1 with a ring of 6, the steriUty 

 was less than 75 per cent. This may indicate that the possibihty of any 

 two segments disjoining is increased when the number of chromosomes 

 in the ring is increased. 



McChntock (1930) has been able to identify the chromosomes involved 

 in the ring in the case of semi-sterile 2, and she has also shown pachytene 

 stages of the ring, which clearly show parasynapsis. Cases in which three 

 chromosomes of the ring pass to one pole and one to the other have been 

 observed, and trisomies have been identified in Zea Mays (Burnham, 

 1930). 



Zea Mays and Pisum sativum are the two plants in which sterility 

 has been most clearly shown to be associated with ring formation, 

 followed by non-disjunction. Semi-sterility inherited in a manner in 

 strict accordance with that in Pisum has been reported in Stizolobium 

 by Belling (1914), but no cytological evidence of a ring has been reported 

 in this case. Ring-forming strains have been found in Campanula 

 (Gairdner and Darhngton, 1930), but the proportion of non-disjunction 

 and the occurrence or otherwise of non-viable gametes has not been 

 reported. 



In Datura Stramonium lines have been investigated which on crossing 

 with a standard Hne (Line 1 A) give 50 per cent, pollen abortion, whereas 

 when crossed with Line 7 (which gives 50 per cent, abortion with Line 1 A) 

 no pollen abortion is observed. However, F^^b between Line 1 and Line 7 

 showed no configurations of four, whereas in J^'s between Line 1 and 

 " B " whites, closed rings of four were frequent, although this cross shows 

 no appreciable sterihty (Blakeslee, 1928, 1929). Blakeslee accounts for 

 the absence of pollen abortion in the latter case by assuming that "like 

 parts are repelled at separation," in which case disjunction will always 

 occur. The presence of pollen abortion in a plant which shows no rings 

 or chains can be explained on the basis of segmental interchange between 

 non-homologous chromosomes if the interchanged parts are assumed to 

 be in a region where chiasmata are very infrequent, or are assumed; to be 

 too small to afEect pairing. It will be seen that where segmental inter- 



