366 AN INTRODUCTION TO ZOOLOGY 



clear, and there are conflicting theories put forward as to the forces 

 that are responsible for the phenomenon. 



The chromosomes themselves take on a variety of forms, spheres, 

 short rods, long rods, V-shapes, Y-shapes, and so on, and different- 

 shaped chromosomes may occur in one and the same nucleus, 

 though each species or tissue usually retains the same arrangement 

 in its mitoses. The next step in the division, i.e. the metaphase, is 

 that each individual chromosome splits longitudinally, that is, in a 

 direction at right angles to the long axis of the spindle. Half of each 

 chromosome, therefore, is apparently attached to the mantle fibres 

 of one aster, and the other half to the fibres of the other aster. This 

 constitutes the essential stage in mitosis, and to which the prophase 

 can be considered as preparatory. In the latter we find that the 

 chromatin is collected up, sorted out into definite pieces, the 

 chromosomes, and these are so arranged that their division and 

 distribution can be carried out as accurately as possible. The 

 remaining two phases can be regarded as, in a way, reconstruction 

 and finishing stages, serving to secure the proper distribution of the 

 chromatin. In some instances there is, as it were, a pushing forward 

 of the splitting process, even as far as the spireme stage, so that from 

 that point on the chromatin and chromosomes when formed are 

 double. Strictly speaking, the term ' ' metaphase ' ' can only be applied 

 when the chromosomes split after being arranged in the equatorial 

 plane, but it will be seen that, whatever method may be adopted, it 

 does not alter the fundamental phenomenon of the sorting and 

 division of the chromatin into equal and similar portions. 



In the anaphase the halves of each chromosome move towards the 

 opposite poles of the amphiaster. It appears as if their role in this 

 is entirely a passive one, and that they are drawn apart by the 

 contraction of the mantle fibres. The groups of chromosomes are 

 carried right to the pole, and so are ready for the last stage. As the 

 groups separate there will be left in the middle region of the spindle, 

 the central fibres, which are now termed the interzonal fibres. 

 These are further added to by other fibres which appear between the 

 diverging chromosomes in the position formerly occupied by the 

 mantle fibres. The origin of these secondary fibres is not clear, but 

 they are also included in the term " interzonal fibres." In plant cells 

 frequently the interzonal fibres thicken in the middle to form what 

 is known as the mid-body or cell plate, and this appears to take 

 part in the formation of the cell wall separating the daughter cells. 

 During this phase there is usually a definite constriction in the 

 cytoplasm of animal cells, foreshadowing its division into two in a 

 plane passing through the middle of the interzonal fibres. 



The concluding stages, the telophase, lead to the transformation of 



