THE PKOBLEM OF MITOSIS. 
589 
at the poles, it is impossible to convert the anti-spindle into 
the spindle without at the same time changing the signs of 
the poles from like to unlike ; and that, if the forces cannot 
be so expressed, the spindle is not a figure formed entii’ely by 
the action of forces at its poles. In the circumstances we 
must accept Meves’ conclusion that : “ Die Strahlungen nicht 
als die sichtbar werdende Wirkung einer in den Centren 
lokalisierten Kraft anzusehen sind; womit aber nicht ausge- 
schlossen ist dass die Centren bezw. Ceutralkorper iiberhaupt 
Einfliisse irgend welcher Art auf das umgehende Cytoplasma 
ausiiben.” And, since the conformation of the spindle figure 
can no longer be regarded as an index of the actions at the 
poles, all early theories that depend for proof upon this con- 
formation must now become mere hypotheses. 
Meves defined theories belonging to the second class by 
saying : Fadentheorien sind solche, nach welchen die wir- 
kenden Krafte ausschliesslich in den Faden ihi’en Sitz haben 
und nach welchen die im Centrum der Strahlungen gelegenen 
Gebilde in erster Linie die Rolle von lusertionsmittelpunkten 
besitzen.” They were, moreover, divided into two sub-classes 
accoi’ding as the kinetic phenomena of mitosis were attributed 
entirely to contraction or to contraction and elongation of the 
rays. Such were the theories of van Beneden, Boveri, 
Driiner, Flemming-, Hermann, 0. and R. Hertwig, Meves, 
Rabl, Solger, and Ziinmermann. We have seen in the intro- 
duction of this paper that objections have been raised against 
these theories, and that independent investigators have 
denied contractility in every portion of the achromatic figure 
in turn. In the circumstances the theories belonging to the 
first sub-class seem to be completely disproved ; and in view 
of the conflicting evidence those belonging to the second 
must be regarded as speculative in that they assume that the 
mantle fibres pull the daughter-chromosomes apart. 
Conclusion. 
Let us now consider our position. We have found that 
none of the theories discussed can be accepted as an adequate 
