254 EVOLUTION AND ANIMAL LIFE 



somes the nuclear membrane has disappeared. The chromosomes 

 soon reach their maximum staining capacity, and appear usually as 

 a collection of rods or bands of deeply staining substance lying free 

 in the cytoplasm. 



" While this is taking place in the nucleus, another series of changes 

 has been gone through by the centrosome and the cytoplasm im- 

 mediately 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, constricts 

 transversely into a dumbbell-shaped figure, and divides into two 

 daughter centrosomes, which at first lie side by side but soon move 

 apart. Around each of them is gradually developed a stellate figure 

 composed of a countless number of delicate fibrils radiating out in all 

 directions from the centrosome as a center. This 'aster' or 'astro- 

 sphere ' is at first small in extent, but grows in size progressively as the 

 two centers move apart, apparently being derived from a rearrange- 

 ment and modification of the threadlike network of the cytoplasm 

 under the influence of the centrosomes. 



' 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 center of one aster to that 

 of the other. This fusiform figure is termed the 'central spindle.' 

 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 dis- 

 appears, a system of fibrils grows out from each astrosphere, which 

 attach themselves to the individual chromosomes. These 'mantle 

 fibers' insert themselves along the chromosomes in such a way that 

 each segment receives a series of fibrils from each pole of the amphi- 

 aster, the two series being attached along opposite sides of the chromo- 

 somes. Under the influence of these fibers, probably by direct pulling, 

 the chromosomes, now bent into V- or U-shaped loops, tend to place 

 themselves in a circle around the center of the spindle, transversely 

 to its long axis, and form the 'equatorial plate.' 



'The changes thus far constitute the 'prophases' of the division. 

 The ' metaphases ' following these consist primarily in the longitudinal 

 splitting of each chromosome and the moving apart of the halves. 

 This longitudinal splitting of the chromosome into two equivalent 

 parts forms the most important act of the whole cell division, and is 



