624 
T. H. MORGAN 
But this very difficulty of proof is evidence of the insufficiency of 
the demonstration derived from this source. 
The best evidence that I have to bring forward against the 
center of force hypothesis is derived from the spiral asters. 
When the eggs of Cerebratulus are drawn out into strings the 
polar asters or the sperm asters or the asters that will form later 
the poles of the spindle are carried out into the elongated mass 
(plate 7, figs. 36-45, and plate 8, figs. 47-54). 
In some cases these fibres are destroyed especially at the outer 
ends, but in most cases they are present and retain their radial 
structure, as shown in fig. 36. This result would seem to mean 
that the fibres of the young asters are relatively rigid, and not 
easily disturbed by the flow of protoplasm about them. But the 
same facts might be interpreted to mean that the centers of force 
that are drawn out in the flowing mass reform the asters wherever 
they move. 
How^ever if the eggs are centrifuged w^hen the large karyokinetic 
figure of the segmentation phase is present the result of the centri- 
fuging is shown by the asters. These are drawn out as the spindle 
or the reconstructed nuclei are carried towards the pole of the egg 
and bent, at many different angles as shown in plate 6, in figs. 32- 
33 and plate 8, figs. 47-53. When the course of one of the poles 
has been oblique, the fibres are thrown into beautiful spiral forms 
as shown in figs. 48, 50. The results give every evidence that the 
fibres are actual fibres that have a real existence in the eggs. 
Their existence may be temporary, but w^hile they last they appear 
to be denser portions of the network. On such an assumption the 
bent course of the fibres here described can be explained. On the 
center of force hypothesis these conditions are inexplicable. 
The fate of the inclosed polar spindle 
In the series of preparations at my disposal I have not been 
able to follow the fate of those eggs in which the polar spindles, 
failing to rise to the surface, divide within the interior of the egg. 
Many problems of peculiar interest are connected with their 
history. Whether such eggs develop parthenogenetically owung 
