186 



KARYOKINESIS. 



Waldeyer), and by the time the spindle has reached the middle of the nucleus, these 

 filaments, now shortened and become distinctly V-shaped, are seen to be arranged in 

 a star-like manner radiating from its equator. This star-like disposition is best seen 

 when the nucleus is viewed in the direction of the axis of the spindle ; the stage is 

 known as that of the aster or monaster (fig. 215). By the time that this stage is 



Pola 



(Cytttsler) 

 ctfbvicticn, sj)h&-es~ 



clear area 

 of nucleus'' 



Fig. 214. DIVIDING NUCLEUS, SHOWING SPINDLE OK ACHROMATIC FIBRES, WITH THE CHROMOSOMES 



ARRANGED A8TRALLY AT THE EQUATOR OF THE SPINDLE. (Rabl.) 



arrived at, and sometimes even while the chromatic filaments still form a convoluted 

 skein, a remarkable change, first noticed by Flemming, is found to have occurred in 

 them, each chromosome having become split along its length into two exactly similar 

 but finer filaments, which remain adherent to one another for a while (fig. 211, g, 

 and figs. 213, 214, and 215). There can be little doubt that this cleavage of the chro- 



acJiromatec,\ 

 spindle- (,, 



Fig. 215. DIVIDING NUCLEUS AT "ASTER" STAGE, SEEN FROM ONE OF THE POLES. 



SPINDLE IS REPRESENTED FORESHORTENED. (Rabl.) 



THE ACHROMATIC 



mosomes is one of the most important of the karyokinetic changes of the nucleus, 

 for the result is that the chromatin is thereby divided into two precisely equal 

 amounts, which pass as the subsequent phases show to the respective daughter- 

 nuclei. 



The next stage in the process of karyokinesis has been termed " metakinesis " and 

 consists in the separation of the chromosomes which have resulted from the longitu- 

 dinal division above described (fig. 211, i,j, fc). The separation usually begins at 

 the apices of the V-shaped loops which as they separate become turned towards the 

 poles of the spindle, while the limbs often remain adherent for a time at the 

 equator of the spindle (fig. 216, A and B), and even after separation remain con- 

 nected across the equator by delicate achromatic uniting filaments (fig. 216, C). 1 



1 It is now agreed by most observers that these " uniting filaments " are not, as was formerly sup- 

 posed, parts of the achromatic spindle, but that they are probably spun out from the chromosomes as 

 these pass towards the poles of that spindle. It is assumed that the spindle-fibres shorten, and that 

 this shortening or contraction tends to draw the chromosomes towards each pole. According to v. Beneden 

 and Rabl, the spindle-fibres are very numerous, and Rabl believes them to be attached to the chromatin 

 granules of Pfitzner, of which the chromosomes are made up, the spindle being thus formed of two 

 distinct cones of achromatic fibres. 



