500 CELL MECHANICS 



brium between attractions and repulsions. This is seen most 

 clearly where two chromatids are paired at the first meiotic meta- 

 phase for a very short distance between two interstitial chiasmata 

 (D., 1936 d). A gap may then be seen between them just as between 

 the sister chromatids in Drosophila. The attraction is never, as on 

 chemical analogy it never can be, absolute in its effect. It is always 

 in equilibrium with the repulsions (which will be considered later) 

 and whereas the repulsions should be a function of area, the attrac- 

 tion should be a function of volume. Smaller attracting parts will 

 therefore lie further apart. Similarly, at particular stages, subject 

 to particular substrate conditions the distinction between primary 

 and secondary attractions may seem to lapse. The distinction is 

 quantitative in cause, but it is, as we shall see, qualitative in effect 

 under the conditions governing metaphase and anaphase movement 

 in all organisms. 



{b) N on-Specific Attractions. The only type of association between 

 non-homologous parts of chromosomes in the nucleus consists of 

 the association of ends. This is found in the prophase of meiosis 

 in two ways. First, there is the special type of association found in 

 the Coccidae (F. and S. H. Schrader, 1923-32). This is perhaps 

 analogous to the formation of a continuous thread in the prophase 

 of meiosis in I eery a and in haploid Triticum monococum. Its 

 causation is unknown. Secondly, there is the special type of 

 arrangement of chromosomes at the zygotene stage described as 

 polarisation. The chromosomes come to lie during leptotene with 

 one or both ends turned towards a particular part of the surface of 

 the nucleus. In some species this movement has nothing to do with 

 the centromere. Thus it applies equally in Chorthippus to chromo- 

 somes with the centromere near the middle or the ends. In other 

 species, the centromere ends of the chromosomes alone are polarised 

 (Mecostethus, Phrynotettix) . Two or even three centres of polarisa- 

 tion may occur in Chorthippus, one of them being the X chromosome 

 which attracts certain other chromosomes specifically. It therefore 

 seems that the attraction is between the chromosomes themselves 

 rather than for something non-specific outside the chromosomes 

 such as the centrosome, as Janssens (1924) and others have supposed 

 (D.,1936^), 



