COEFFICIENT OF HYBRIDITY 271 



inversions are indicated by the relative frequency of double as well 

 as single bridges, and in determining the position of an inversion 

 the relative frequency of second to first division bridges should be 

 correlated with the length of the bridges themselves. Where the 

 fragment is a-shaped there has been a chiasma in the straight 

 segment distal to the inversion. 



In animals and plants with several inversions it is not always 

 possible to define each one. But it is possible to obtain quantitative 

 data showing what the comparative biological importance of 

 inversion hybridity is in different forms. We can calculate the 

 approximate frequency of crossing-over between relatively inverted 

 segments by adding together the frequencies of bridges at the first 

 and second division per mother cell (not per cell), counting at the 

 same time all double first division bridges as six, instead of two, 

 since these, resulting from complementary crossing-over, presumably 

 represent equal numbers of reciprocal crossings-over which do not 

 give bridges, and disparate crossings-over which give only one bridge 

 for two. This calculation will not allow for the corresponding result 

 in second division bridges, nor will it allow for triple crossing-over 

 in inversions ; these, however, will always be very rare. 



The inversion crossing-over frequency obtained in this way is 

 itself an index of the effect of the hybridity on the fertility of the 

 organism. By dividing this value by the average chiasma-frequency 

 per cell we can obtain a coefficient of hybridity in regard to inversions. 

 With this datum comparison may be made of the hybridity equili- 

 brium in different natural populations (D., 1936 d). The equilibrium 

 is highest in plants that are largely propagated by asexual means, 

 lowest in plants with self-fertihsation, and intermediate in the 

 insect populations that have been examined. 



(c) Later Behaviour of Dyscentric Configurations. With regular 

 disjunction a short chromatid bridge is broken at the anaphase at 

 which it appears, a long one may persist until it is broken by the 

 separation of pollen grains or spermatids. The acentric fragment is 

 entirely passive. It may either lie free on the plate or be strangled 

 by the bridge chromatid coiling round it. Such a fragment may be 

 carried into the nucleus, and has been seen lagging in the pollen- 

 grain mitosis in Podophyllum (D., 1936 b). 



