498 CELL MECHANICS 



fore almost entirely lapsed. The contrary change may occur in the 

 complete lapse of the residual attraction : this is probably, as we 

 saw earlier, a corollary of a reduction in the primary attraction. 

 The B chromosomes of Zea Mays, which are inert and over-con- 

 densed at metaphase, show by their frequent torsion pairing a 

 reduced primary attraction and by their complete absence of juxta- 

 position at mitosis (Fig. 102) a reduced secondary attraction. 



The association of centromeres in pachytene cells does not allow 

 us to infer their own particular mechanical properties at this 

 stage. It is clear, of course, that the centromere is not exerting 

 the repulsion shown in anaphase movement and terminalisation 

 [q.v.). But we cannot say whether it exerts an attraction on other 

 centromeres in the pachytene nuclei because chromomeres adjoining 

 i+ may be responsible for the fusion or pairing that it undergoes. 

 This does not apply to the polytene nuclei wt ere all the centromeres 

 fuse, or to diakinesis in male Drosophila, for the sudden change 

 that takes place between diakinesis and metaphase leaves no doubt 

 that here an attraction has been followed by a repulsion. 



There is, moreover, some evidence in Agapanthus and Staurodenis 

 that at the pachytene stage an attraction exists between the 

 centromeres of non-homologous chromosomes. This does not 

 mean that the attraction is non-specific but rather that all centro- 

 meres are homologous, i.e., their common forms and functions are 

 inherited from a common progenitor, a single ancestral centromere 

 (D., 1935, h and g). 



These special properties express three important principles of 

 chromosome mechanics. First, the anomalies of specific attraction 

 at meiosis and in the permanent prophase oi Drosophila are associated 

 with other anomalies of movement. At diakinesis the centromeres 

 of paired chromosomes, and in the permanent prophase the centro- 

 meres of all the chromosomes, are closely associated instead of 

 repelling one another as the centromeres do at these stages in normal 

 circumstances. This correlation of behaviour in different cells 

 shows the importance of a substrate effect in chromosome movements. 

 It shows that the attractions of chromomeres and centromeres are 

 conditioned by a common substrate and therefore, like other pro- 

 perties of the chromosomes, subject to physiological control. 



