1900] ACHROMATIC SPINDLE OF OSMUNDA 369 
rounded and ill defined, and its breadth relatively great in com- 
parison with its length. But in all cells from which the nuclear 
membrane has disappeared, the spindle is seen to be consider- 
ably narrower, and at the same time longer and more sharply 
pointed. To explain these phenomena it seems necessary to 
suppose that a pressure, exerted by the spindle fibers upon the 
nucleus, is sustained by the nuclear membrane, and that when 
the membrane finally gives way, its collapse is attended by a 
sudden diminution of the diameter of the spindle ( figs. 7, 8) 
together with a corresponding increase in length. At the same 
time, the pressure of the fibers crowds the chromosomes close 
together. If the fibers be conceived of as curved elastic rods, 
this action becomes intelligible; that they are rigid enough to 
exert pressure is probable from the fact that the second spindles 
frequently cause a widening of the ends of the mother cell, 
Pushing out the wall so as to give it the form in section of a 
figure 8. 
One effect of the sudden collapse of the nucleus is that the 
chromosomes, which were all necessarily on the inner side of 
the spindle fibers, are most of them forced to the outside. No 
distinction of central and mantle fibers was possible, though 
Very great care was exercised at this point of the investigation. 
It is clear from fig. 9 that the original fibers, which run merid- 
lonally, still remain so after the separation of the chromosomes. 
€peated searching under a magnification of 2250 diameters 
failed to reveal any fibers other than those which run from pol 
‘0 pole, or any thickening of the fibers towards the poles, as if 
‘ome portion attached to the chromosome were undergoing con- 
fraction. How then are the chromosomes propelled ? Inasmuch 
°S there are no fibers discoverable by which they can be pulled, 
'S it not possible that they have a power of motion in them- 
Selves ? 
pole 
This hypothesis is not absurd. It is quite as reasonable to 
ume an automobility of the chromosemes as @ contractility of 
; ‘pindle fibers. The initiatory separation of the — 
2 i + e : n 
mo Pairs by longitudinal fission implies a power of moveme 
