THE CELL AND THE TISSUES. 



towards a common centre, around which, but removed some little 

 distance, they become arranged. The enclosed clear space is the 

 polar field. 



The centrosome early undergoes division, when not already double 

 within the cytoplasm, and lies close to the nuclear membrane. The 

 new centrosomes, enclosed within the attraction spheres, soon begin 

 to separate, each receding from its original position through an arc 

 of 90 towards the opposite poles of the cell. 



During the formation of the skeins and the division of the centro- 

 somes the nuclear membrane disappears and the chromosomes lie 

 free within the cell-protoplasm. Coincidently with the foregoing 

 changes delicate striae make their appearance around each centro- 

 some, the radiating figure thus produced constituting the amphi- 

 aster. The rays bridging the space separating the centrosomes are 

 so disposed that together they present a double cone, whose apices 

 are directed towards the poles of the future new nuclei, as determined 

 by the new centrosomes ; these achromatin figures constitute the 

 nuclear spindle, and occupy the polar field. The cromatin fibrils, 

 now staining intensely, grow thicker and shorter, 

 and arrange themselves so that the closed ends 

 of the loops encircle the polar field, giving rise, 

 when seen from the surface, to the wreath ; seen 

 from the side, however, the loops or V's appear 

 as radiating fibrils, and constitute the 



(4) Mother-star, or aster : the apparent dif- 

 ferences, therefore, between the wreath and the 

 aster depend upon the point of view, and not 

 upon variations in the arrangement of the fibres. 

 The completion of these preparatory changes 

 marks the termination of the prophases ; the suc- 

 ceeding metaphase is initiated by an important 

 change leading to the reapportionment of the 

 chromatin. 



(5) Each of the loops, or chromosomes, un- 

 dergoes longitudinal cleavage, splitting up 

 into double the number of segments : these are 

 now entirely rearranged during the anaphases, 

 the first step being: 



(6) A rapid separation into two groups, pass- 

 ing towards the poles of the future new nuclei, 



as indicated by the foci of the nuclear spindle. Around these points 

 as centres, a delicate radial marking the polar striation appears. 

 The halves of the longitudinally cleft fibrils are so disposed that one 

 of each pair of sister-segments passes along the guiding lines of the 



GP 



Rearrangement and 

 cleavage of V-segments : 

 A , from the side . B. 

 from the polar field, P ': 

 GP, anti-pole. (After 

 Rabl-Schieffer decker. ) 



