THE STRUCTURE OF CELLS 



stances do exist in the nucleus, as they certainly do in the cyto- 

 plasm. For in the case of, e.g., the micronuclei of infusoria, the 

 whole spindle is entirely intra- nuclear, the cytoplasm apparently 

 not furnishing, at least directly, any part of it. 



With the congregation of the chromosomes to form the 

 equatorial plate, the first stage or Prophase of division terminates. 



The equatorial plate, or aster, stage is often one of relatively 

 long duration ; so much so that it may even happen that some of 

 the signs of cytoplasmic activity may fall into temporary abeyance. 

 For example, the astral radiations outwards from the centrosomes 

 may cease to be visible at this stage (e.g. in Pellia), though they 



FIG. 0. 



Stages of the mitosis in the micronucleus of Paramorclvm, 

 liowing the "pole plates"; true centrosomes are not present. 



Affill- T? Uuft-lltir, \ 



(After R. Hertwig.) 



reappear later on. The nuclear wall commonly, though not 

 always, disappears whilst the chromosomes are collecting at the 

 equator, and the nucleolus or its fragments, if they have not 

 previously disintegrated, now are no longer recognisable. 



The individual chromosomes often, but by no means always, 

 assume the shape of a V, with the apex turned centrally in the 

 equatorial plane. Each one is supported by fibres of the achromatic 

 spindle which run from the poles, and terminate on the chromosomes 

 at the equator. The chromosomes next split longitudinally, and 

 this partition forms the commencement of the stage known as the 

 Metaphase. The two daughter halves rapidly diverge, being guided 

 by the spindle fibres towards the poles. During their divergence 



