590 
0. F. U. MEEK. 
explanation of mitosis ; for those that are not actually dis- 
proved appear to be either partly or entirely hypothetical. 
In the circumstances there is only one course of action left 
for us to adopt. We must try to discover if there is any 
generalisation upon which all these theories are in agreement, 
or that may be regarded as having been established by the 
reasonings of their supporters. It is at once evident tliat 
there is such a generalisation, viz. that the mitotic spindle 
is not a figure formed entirely by the action of 
forces at its poles. Hartog’s arguments have proved that, 
if this is denied, the spindle cannot be formed by known 
forces. Gallardo and Rhumbler have been foi’ced to accept 
this proposition ; although the latter, in doing so, has sacri- 
ficed the proof upon which he sought to establish his theory. 
Lawson’s interpretation of mantle-fibre formation involves 
this proposition ; and we have seen that the older theories 
either admit it, or are disproved if they do not admit it. 
This, therefore, and this alone, is the basis upon which 
we must raise the superstructure that will eventually explain 
these phenomena. 
Is there a second generalisation that can be regai'ded as 
established ? There is none ; for all others are either hypo- 
thetical or dependent upon personal observations that are not 
accepted by all investigators. There is, however, one con- 
clusion that must be drawn from the proposition that we 
have established, viz. that the spindle figure cannot now 
be regarded as an index of the actions at the poles : at 
present we have no means of determining how far these 
actions, if existing, are limited or counteracted by the other 
forces whose presence we have been compelled to admit; 
and, until this is known, no theory depending entirely for 
proof upon the conformation of the spindle figure can be 
regarded as other than hypothetical. For this reason the 
utility of models in the immediate future seems to be 
doubtful. 
In the circumstances we must collect data, and not attempt 
to furnish an explanation of mitosis until we have been able 
