STUDY OF CELL MECHANICS 



519 



That the pecuhar bent fibers are not caused by the movement 

 of the division center itself, in the case of the gastropods, is shown 

 by the figures of both Mark and MacFarland. I reproduce figure 

 66 from Mark's paper in figure F. Here it will be noted, first 

 that the egg aster does not lie in a normal plane, but has been 

 shifted to one side. And secondly, that the rays themselves run 

 forward and then backwards. The figures of MacFarland show 

 the same thing for the rays in the eggs of Pleurophyllidia. I 

 have explained how such a course of the fibers, in the case of the 

 sea urchin, is incompatible with the view that the asters them- 

 selves move first. It is clear that, as Conklin has shown, the 





moA'ements of the protoplasm are responsible for the bending 

 of the rays. The reason that the twisting of the rays has been 

 described so often for gastropods doubtless lies in the fact, that 

 as Linville has pointed out, the egg aster persists for an unus- 

 ually long time after the second polar body is given off. Byrnes 

 has given figures which show very plainly the persistence of the 

 rays until the pronuclei are uniting. 



Coe ('99) in working on the maturation and fertilization of 

 Cerebratulus observed many cases where the fibers running from 

 the centrosphere were bent. The most interesting case, per- 

 haps is that shown by the author in figure 37, plate 21, which I 

 have copied in figure G. A glance at this figure will show that 



