154 FOOT-NOTES TO EVOLUTION. 



While this is taking place in the nucleus, another 

 series of changes has been gone through with by the 



centrosome and the cytoplasm immedi- 

 Division of the ^^^j^ surrounding it. We have already 

 centrosome. indicated the presence of the centrosome 



as a minute spherical structure lying at one side of the 

 nucleus. This body assumes an ellipsoidal form, con- 

 stricts transversely into a dumbbell-shaped figure, and 

 divides into two daughter centrosomes, which at first lie 

 side by side but soon move apart (Fig. 5, A). Around 

 each of them is gradually developed a stellate figure 

 composed of a countless number of delicate fibrils radi- 

 ating out in all directions from the centrosome as a 

 centre. This " aster " or "astrosphere " is at first small 

 in extent, but grows in size progressively as the two 

 centres move apart, apparently being derived from a re- 

 arrangement and modification of the thread-like net- 

 work of the cytoplasm under the influence of the cen- 

 trosomes (Fig. 5, B and C). 



Between these two asters, which lie a short distance 

 apart and at one side of the nucleus, a spindle-shaped 



system of delicate fibrils may often be 

 The spindle. ^^^^ ^^^^ stretching from the centre of 



one aster to that of the other. This fusiform figure is 

 termed the "central spindle" (Fig. 5, D). The two 

 asters, together with the central spindle, form what is 

 termed the "amphiaster " or the "achromatic " portion 

 of the karyokinetic figure. The two series of changes 

 in nucleus and cytoplasm, which have thus far gone on 

 apparently independently of each other, now become 

 closely interrelated in that, as the nuclear membrane 

 disappears, a system of fibrils grows out from each 

 astrosphere which attach themselves to the individual 

 chromosomes (Fig. 5, D). These " mantle fibres " insert 

 themselves along the chromosomes in such a way that 



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