CELL DIVISION 105 



organ now makes its appearance surrounded by a con- 

 densed or highly granular area of cytoplasm known as 

 the attraction sphere, which is to become the "polar field." 



During these changes the nuclear membrane has im- 

 perceptibly disappeared, leaving the chromosomes free 

 in the cytoplasm. The chromosomes in the meantime 

 have uniformly assumed a V-shape and arranged them- 

 selves about the polar field with the apices toward the 

 centre. It now makes considerable difference from which 

 direction one views the groups of chromosomes, for at the 

 same time that this adjustment has been in progress the 

 centrosome has divided and its halves have gradually sepa- 

 rated, so that we have now a "polar field" and a "hypo- 

 polar field," each occupied by a centrosome from one to 

 the other of which delicate filaments of linin extend, form- 

 ing a peculiar object known as the "nuclear spindle." 



If the group of chromosomes be viewed from a direc- 

 tion corresponding to the polar or hypopolar fields, one 

 sees those bodies with their apices directed toward the 

 centre, their ends outward, forming an appearance some- 

 times compared to a wreath, but usually to a star, and 

 spoken of as the aster or "mother-star." If, on the other 

 hand, they are viewed from a point at right angles to the 

 axis of the nuclear spindle, it will be found that they all 

 lie in a plane perpendicular to the axis of the spindle, 

 known as the "equatorial plate." 



2. The Metaphase, or Division of the Chromosomes. 

 The metaphase begins with a longitudinal division of 

 each chromosome. Here we find the first indication of 

 what is to be the final outcome of the process. Each 

 chromosome has given rise to two smaller chromosomes 

 of equal value, and one-half of each is to enter into each 

 of the newly forming nuclei. The chromosomes next 

 manifest a change of position. 



3. Anaphase, or the Separation of the Chromosomes. 

 Beginning at the centre, the apical portion of each half- 

 Chromosome moves from its fellow and directs itself 



