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- 
ately surrounding it. We have already 
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, 4). 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 
made out, 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, .). These “ mantle fibres” insert 
themselves along the chromosomes in such a way that 
Division of the 
centrosome. 
The spindle. 
