CELLS IN DIVISION 



able to exert tractive effort on the chromosomes, as is shown by the 

 remarkable phenomenon of the 'pre-metaphase stretch' in Mantids. 

 It is difficult to explain the phenomena of metaphase without postulat- 

 ing a state of balance between opposing forces, though in the succeeding 

 section (p 126) Swann suggests that it may be misleading to apply our 

 usual macroscopic conceptions of elastic tensions to the mitotic spindle. 

 It appears that these equihbria act both along and transversely across 

 (OsTERGREN^^^) the spindlc. The random metaphase movements of 

 the chromosomes in chick and mammalian cells at metaphase 

 (Lewis ;^^^ Hughes and Swann"^) are presumably an expression of 

 this longitudinal balance, though these probably do not occur in all 

 cells. The equilibrium position of chromosomes on the spindle at 

 metaphase is not always median; Ostergren^*** has found that in two 

 triploid organisms [Anthroxanthum, a grass, and the Urodele Triton)^ 

 at meiotic metaphase the trivalents are disposed nearer that pole of the 



Figure 45 Expulsion of clumped autosomes from 

 spindle in spermatocyte in harlequin lobe of 

 testis of Mecistorhinus melanoleucus during meta- 

 phase I. (sex chromosomes in black) x 1500. 

 From ScHRADER^*^ {By courtesy, Chromosoma) . 



spindle towards which they present two centromeres (p 125). These 

 transverse equilibria are expressed in several ways; there is a tendency 

 for bodies to be extruded from the spindle ; it is thus kept free of cyto- 

 plasmic granules, and the arms of long chromosomes are usually out- 

 side it. Factors intrinsic to the chromosomes modify this centrifugal 

 tendency. Autosomes and sex chromosomes do not behave in the same 

 way, especially if they differ in the timing of their cycles of nucleination. 

 Sometimes, as in the Mantid Humbertiella (Hughes-Schrader^^i) it is 

 the X-chromosome which is expelled, while in the 'harlequin lobe' of 

 the testes of some Pentatomids (Schrader^*^) the clumped autosomes 

 are displaced, in Mecistorhinus to such a degree that the cell outline is 

 distorted (Figure 45). An extreme instance of the differential behaviour 

 of chromosomes on the spindle is provided by the monocentric first 

 division of the spermatocytes of the fungus-fly Sciara, where paternal 

 chromosomes move outwards at anaphase and maternal ones inwards 

 (Metz^*^^**) (Figure 46). These examples of aberrant chromosomal 

 behaviour are not exhaustive. There are always individual instances 

 which cannot be fitted into any attempted general theory of chromo- 

 some movement. It is possible to pile on subsidiary theories, though 



117 



