14 THE CHROMOSOMES 



to the conclusion that, whatever their relation to 

 the spindle when present, they are at any rate not 

 essential to the division process. Where centro- 

 somes and asters are present (as in the majority, 

 but by no means all the Metazoa) an apparent 

 spindle ifiay form between them outside the nuclear 

 membrane. In this case when the membrane disap- 

 pears this structure (which is a good deal smaller 

 than the final spindle and can be called the central 

 spindle) moves into the middle of the nuclear area. 

 The nuclear sap then apparently undergoes rapid 

 gelation round the original central spindle so as to 

 increase its volume considerably. Thus a compoimd 

 spindle is formed which differs from the previous 

 types only in having a central element not formed 

 of nuclear sap but of extranuclear cytoplasm.^ 



The development of this central part of the spindle 

 outside the nucleus in some cases has proved very 

 confusing, since it has led to the conception of the 

 chromosomes attaching themselves to a preformed 

 spindle ; actually they are associated with the true 

 spindle elements as soon as the latter are formed, 

 and are probably never attached to the central 

 element. 



3. Metaphase 



At the end of prometaphase (the period in which 

 the spindle is formed) the chromosomes are * at- 

 tached ' — the term is misleading, but has to be 

 retained — to the spindle in the region of the equator, 

 that is to say equidistant from its two ends. The 

 arrangement of the chromosomes at metaphase 

 depends on a number of factors, (1) whether a 

 central spindle element is present or not, (2) the 

 number of the chromosomes and (3) their sizes. 

 Where the chromosomes are very long or a large 

 central element exists, as in the dividing leucocytes 

 of Salamandra,* all the chromosomes are arranged 

 round the periphery of the equator, irrespective of 



