616 DR A, ANSTRUTHER LAWSON ON 
observable, as it 1s now in close contact with the majority of the chromosomes, 
Although the membrane cannot now be traced and observed as a definite object—being 
a mere differentiated plasmatic film of unmeasurable thickness,—I think the events 
that follow justify the assumption that it completely envelops each bivalent 
chromosome, for these latter bodies are no doubt still saturated with nuclear sap. 
But the main point of interest here is not so much the actual demonstration of the 
plasmatic membrane closely applied to the saturated chromosomes, but the drawing in, 
with the membrane, of the cytoplasmic fibrils until they reach the chromosomes, 
This I consider to be clearly demonstrated in figs. 29, 30, and 31. As I have already 
stated in another work (Lawson, 19118), this offers a much more rational explanation 
for the attachment of the spindle fibrils to the chromosomes than the generally accepted 
view that the spindle fibrils push into the nuclear cavity and attach themselves with 
their free ends to the individual chromosomes. 
If, as I have attempted to demonstrate above, the cytoplasmic fibrils which 
eventually form the achromatic spindle are expressions of lines of tension, then it 
follows that the positions of these lines would shift as the nuclear membrane receded, 
and later, as this membrane envelops the bivalent chromosomes, each of these 
structures becomes furnished with a system of fibrils. ‘The conical-shaped sheaves of 
fibrils which constitute the multipolar figure. have frequently been described as 
approaching one another and coalescing in two groups to form the bipolar figure. The 
difficulties here are obvious, and the facts available do not sustain this idea. The 
cytoplasmic reticulum seems not to be disturbed in the slightest degree by such a 
coalescence. No real evidence has been recorded to show that a lateral movement and 
fusion of the cones really occurs. On the other hand, if we accept the more rational 
interpretation that the fibrils are drawn-out threads of cytoplasm and merely represent 
lines of tension, the diticulties of understanding their movements and apparent shifting 
of position become very much lessened. For, as I have pointed out elsewhere 
(Lawson, 19118), the apparent change in position is due to the relaxing of the tension 
along certain lines and the establishment of new tensions along others. In other 
words, while the lines of tension do shift, the fibrils themselves do not. These latter 
withdraw or reappear according to the shifting of the position of the nuclear membrane 
—the latter being a continuation of the same plasmatic substance. The manner in 
which this shifting results eventually in the bipolar arrangement has been fully 
described in my last paper (Lawson, 19118), and a further repetition is unnecessary. 
I shall only add that the main conclusion there stated has been fully sustained by the 
present investigation. . 
It seems evident, then, that the results produced by nuclear osmotic changes offer 
a fair and rational explanation of three very important phases of mitosis. First, the 
origin and formation of the fibrils which constitute the achromatic figure. Second, 
the attachment of the fibrils to the chromosomes. And third, the movements associated 
with the resolution of the multipolar figure into a bipolar arrangement. In these 
