SECTIONAL TRANSACTIONS.— K. 423 



Prof. Dame Helen Gwynne-Vaughan, G.B.E. — Incompatibility 

 (lo.o). (Openers of discussion : Dr. M. J. SiRKS, Mr. W. J. C. 

 Lawrence and Dr. M. A. P. Madge.) 



It has long been recognised that particular matings may fail, even when 

 the sexual apparatus is efficient and the gametes are able to function in 

 other unions. Often, in organisms with both male and female cells, self- 

 fertilisation, though not mechanically prevented, proves rare or impossible. 

 Such forms are said to be self-incompatible. Incompatibility may extend 

 beyond the individual to the variety or group of varieties ; a condition of 

 cross-incompatibility then exists. This state of affairs has been little 

 studied in animals, though it has been recognised in the sea squirt. In 

 higher plants, and especially in fruit trees, it is well known and has been 

 shown to depend on appropriate genes. In fungi, also, incompatibility 

 is determined by the nuclear content of the cell ; it is here associated with 

 the disappearance of normal fertilisation and with the problems incidental 

 to the invasion of the land. Incompatibility, like the separation of male 

 and female organs in different individuals, is a means of ensuring exogamy. 



Prof. F. A. E. Crew. — The genetical aspect of crossing over (10.45). 

 (Openers of discussion : Prof. R. A. Fisher and Dr. F. W. 

 Sansome.) 



Dr. C. D. Darlington. — Mechanism of crossing over (11.20). 

 (Opener of discussion : Dr. P. C. Roller.) 



Two important changes take place at meiosis : first, reduction in the 

 number of chromosomes by the segregation of partners, and secondly 

 crossing-over between those partners. Crossing-over takes place as a 

 result of a torsion developed in the paired chromosome threads. This 

 torsion twists the threads round one another until an equilibrium is reached 

 like that in spun wool. The chromosomes divide, the equilibrium is upset 

 and the daughter threads break at opposite points, untwist and rejoin : 

 crossing-over has taken place. 



The result of the division of the chromosomes is that the partners fall 

 apart, attraction being limited to pairs. The result of their crossing-over 

 is that the partners are nevertheless held together at the points of crossing- 

 over or chiasmata. All the later association of the chromosomes and hence 

 their segregation and reduction depends on these chiasmata. 



Crossing-over has always been understood to perform the useful but not 

 indispensable function of recombining hereditary differences in sexual 

 reproduction. We now learn that it underlies sexual reproduction itself, for 

 without crossing-over the necessary reduction and segregation of chromo- 

 somes cannot take place. 



Mr. E. B. Ford. — Genetical control of development (12.0). (Opener 

 of discussion : Dr. J. S. Huxley.) 



Afternoon. 

 Miss G. N. Davies. — The effect of zinc sulphate on dwarf French beam 

 (2.15). 

 In connection with water pollution by lead mines in which zinc is also 

 present, a series of culture experiments has been carried out to ascertain 



