CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 319 



stage closely attached to the surface layer of the cell; the nucleo- 

 lus of the germinal vesicle is carried to the centripetal pole of the 

 egg where it lies with the lightest substances of the cell, but the 

 sperm nucleus may be found now in the cytoplasm, now in the 

 yolk and again on the border between the two, thus showing 

 that it is not moved to any great extent by centrifugal force 

 and that yolk or cytoplasm may stream past it without much 

 altering its position (figs. 3-18). If the sperm nucleus were free 

 to move according to its specific weight it would always occupy 

 some constant position either in the cytoplasm, in the yolk, or 

 in the region between the two; and the fact that it is not found 

 in any constant position with regard to cytoplasm or yolk shows 

 that something prevents its free movement. It seems, probable, 

 therefore, that after the sperm nucleus has entered the egg it 

 becomes attached to the cell fi'amework by astral filaments which 

 radiate from the sperm centrosome and which are so fine that 

 it is difficult to see them and yet so strong that, although all the 

 movable substances of the egg may flow past, the sperm nucleus 

 is not torn from its moorings. 



After the maturation spindle of the normal egg has reached its 

 metaphase it becomes closely attached to the egg surface at the 

 animal pole by one of its asters and thereafter this attachment 

 can not be broken by the strongest centrifuging to which the 

 eggs were subjected. Figures 5 and 6 represent such eggs in 

 which yolk has been driven to the animal pole and cytoplasm 

 to the opposite pole, but the maturation spindles are so firmly 

 anchored to the surface layer of the egg that they can not be 

 pulled away, even though the spindle itself is stretched in length ' 

 and the egg surface is indented, apparently by the pull of the 

 spindle upon it. Since this attachment of the spindle to the egg 

 surface occurred in this case before centrifuging there is no 

 doubt that this point of attachment represents the true animal 

 pole of the egg. 



Figures 9-12 are all from the same experiment; the eggs were 

 centrifuged in the prophase before the first maturation spindle 

 had become firmly attached to the surface of the egg, conse- 

 quently the spindles were carried into the interior. The eggs 



