172 Mot tier.—Nuclear and Cell Division 
not truncated at the ends, but somewhat attenuated or rather 
of a uniform diameter throughout. On entering, some pass 
straight toward the centre of the nucleus, while others diverge 
toward the sides. As these fibres approach from opposite 
sides of the nucleus, they tend to collect the chromosomes 
into an irregular mass in the equatorial region, where they 
finally form the nuclear plate (Figs. 4, 5, 6). Certain of the 
fibres coming from opposite sides seem to unite at their 
ends to form the continuous spindle-fibres which extend 
from pole to pole; others fasten themselves to the chromo¬ 
somes, and still others diverge toward the nuclear wall in the 
equatorial region. In the mature spindle, therefore, the fibres 
present the following orientation : those radiating from the 
poles, the continuous spindle-fibres extending uninterruptedly 
from pole to pole, those running from the poles to the chro¬ 
mosomes, and the fibres which diverge from the poles toward 
the equatorial region and end in the cytoplasm. Of the 
latter there are fewer in the mature spindle than at an 
earlier stage. In addition to the phenomena just described, 
several others may now be mentioned. Sometimes (Fig. 3) 
the nuclear membrane is apparently pushed in at the poles 
where the spindle-fibres penetrate. This may be due in 
part to a slight shrinkage. Up to this stage the nuclear 
membrane is unbroken. As the spindle-fibres enter the 
nucleus, its membrane begins to disappear at the poles, and 
very soon it is no longer to be recognized as such at those 
points, while at the sides it remains almost unchanged until 
a later stage or after the spindle is fully formed (Figs. 4, 5, 6), 
when all traces of nuclear membrane finally disappear. Thus 
the spindle, with the exception of the polar radiations, lies 
within the nuclear cavity, its fibres, however, being of cyto¬ 
plasmic origin. How far any nuclear substance contributes 
to the formation of the spindle is difficult to decide. 
As previously mentioned, the polar radiations become much 
less pronounced and apparently fewer in number as soon as 
the spindle-fibres enter the nucleus. The centrosomes also 
become smaller (Fig. 5), being frequently almost concealed 
