i82 STRUCTURAL HYBRIDS 



coiling sometimes leads to abnormalities corresponding to the non- 

 homologous pairing found at meiosis. Thus an intercalary unpaired 

 piece of chromosome may coil round itself and prevent regular 

 association. On the other hand, where an inversion is present the 

 coiling seems to be a necessary preliminary to the formation of a 

 reversed loop. 



Mechanically therefore we can look upon the salivary gland 

 nuclei as in a state of permanent prophase in which two special 

 properties are developed to excess. These are the properties of gene- 

 reproduction and gene-attraction. Since, as we shall see elsewhere, 

 reproduction must itself be regarded as a function of attraction (for 

 materials in the substrate) this is not surprising. The result is to 

 produce an enormously greater bulk of the gene materials, or at 

 least of those which are physiologically active, a bulk comparable 

 with that found in glandular cells elsewhere which are polyploid, 

 but continue to undergo mitosis. 



Abnormalities of pairing arise in homozygous individuals of 

 Drosophila in two ways. First the reduplication of segments leads 

 to pairing within the chromosome as well as between chromosomes 

 (Bridges, 1935). Secondly, owing to internal torsion an intercalary 

 unpaired segment may become coiled round itself and then this coil 

 breaks its normal pairing (KoUer, 1935). These abnormalities, 

 like the normal behaviour in structural hybrids, agree well with 

 what we know of the pachytene stage of meiosis in similar organisms. 

 They therefore extend our knowledge of structural change, of the 

 hybrids resulting from it, in nature and in experiment, and of the 

 effect of this hybridity on the processes of germ -cell formation. 



