24 CYTOLOGY CHAP. 



ever, no mechanical theory of contraction and expansion appears to be 

 capable of giving a satisfactory explanation of all the facts, and 

 consequently many cytologists look upon the rays of the aster and the 

 spindle " fibres " merely as the expression of lines of force emanating 

 from the centrosomes as centres. An analogy for this view is found in 

 the position taken up by iron filings in a bipolar magnetic field. A very 

 serious objection to this theory, however, is that the lines radiating 

 out from the two centrosomes frequently cross each other, and this 

 is incompatible with a system of lines of force. This is illustrated 

 incidentally in Fig. 33, D. 



The mechanism by which the achromatic figure brings about the 

 movements of mitosis must therefore be for the present admitted to be 

 quite unknown. An account of the whole problem of the achromatic 

 figure, much fuller than attempted here, will be found in Wilson's text- 

 book The Cell, and a discussion of the physical and mechanical problems 

 involved in mitosis is given by Meek (1913). 



D. AMITOSIS 



In amitosis or direct division, the nucleus, without departing from 

 the resting structure, divides by simple transverse fission. In the 

 simplest case it first changes from a sphere into a dumb-bell or hour- 

 glass shape (Fig. 12), and then becomes nipped across at the constriction 

 to form two separate nuclei. In other cases the nucleus produces lobes 

 which may become constricted off from the parent nucleus to form 

 independent nuclei. In this mode of nuclear multiplication there is no 

 chromosome formation, and apparently no mechanism to ensure that 

 the daughter nuclei are each supplied with one of each of the chromatin 

 elements of the mother nucleus, as is provided for by the longitudinal 

 division of the chromosomes in mitosis. Therefore if each product of 

 amitotic division, at least amongst the Metazoa and Metaphyta, were 

 able to form a complete set of perfect chromosomes, it is plain that we 

 should have to modify greatly the theory of chromosome continuity 

 and differentiation outlined above, or else postulate some at present 

 unknown mechanism for the precise partition of the chromatin elements 

 in amitosis. This would be especially the case if it were shown that 

 normal gametes could be produced from the descendants of cells which 

 have divided amitotically, though it is conceivable that tissue cells could 

 survive and multiply even though lacking some of the chromosomes or 

 chromosome constituents. 



The mere occurrence of amitosis therefore is of little significance 

 unless it can be shown that nuclei produced in this way can afterwards 

 proceed to complete chromosome formation and normal mitosis. In 



