Divisions in Vicia Faba . 
851 
It remains to be questioned why the cross attachments formed at this 
stage are stronger, as the above implies, than the attachments formed 
between sister chromosomes (PL LXII, Fig. 30), or indeed any near neigh¬ 
bours in the prophase and early metaphase. It is probably a relevant con¬ 
sideration that the chromosomes, crowded together at the narrow end 
of the spindle, seem to be in contact under pressure, whereas they lie loosely 
side by side in the earlier stages. 
As far as we can see, though fission is begun in the telophase, it is not 
complete till after the spireme is formed, for, even in the early spireme 
stages (Fig. n), the thread in Vicia is made up of alternate double and 
apparently single portions, and in Figs. 5 and 8 a clear relation seems 
to exist between the cross attachments and the points of fission. It must 
here again be borne in mind that the thread in the regions of cross attach¬ 
ment is comparatively tense, and that it is comparatively slack between 
these points, so that there the sides of a split thread might readily fall 
together. The possibility thus remains that the cross attachments do not 
cause the split, but only make evident a fission already accomplished. 
Mechanism of Mitosis. 
It is difficult again to obtain useful evidence as to the mechanism which 
brings about the separation of the daughter chromosomes, but the stages by 
which this is accomplished in Vicia seem fairly simple. The chromosomes 
become attached by one end to the spindle, they lie lax for a time along it, 
they are swung out at right angles to the spindle axis, and the separation of 
the two halves of the attached end begins. The halves remain in contact 
at the free end for a considerable time, so that the daughter chromosomes 
form first an acute and later an increasingly obtuse angle, till they come to 
lie almost along the same straight line. By this time they are quite free 
one of another; they show no decrease in length till they approach the 
poles, when they are shortened and thickened, forming the dense mass 
described. The contraction of the chromosomes at this stage is no doubt 
due to the continued action on their lagging ends of whatever force is 
responsible for their movement along the spindle. 
It is well known that after good fixation the spindle fibres are often 
less conspicuous than when inferior fixatives are used. This fact is 
evidence against the recognition of the fibres as definite cell entities: 
Farmer and Moore (2) have regarded them as protoplasm modified by 
the forces at work in the cell, and we are led to suggest that an important 
part may be played by currents of altered cytoplasm in Angiosperms, much 
as these have been suggested (Fraser and Welsford (4)) to be responsible for 
the changes taking place in the cytoplasm of the ascus among Fungi. 
If it be credible that such an alteration is in the direction of greater 
osmotic activity, then we should have a mass of osmotically active substance 
