STUDIES WITH THE CENTRIFUGE 31 



week and produced normal adults. In one case it is certain that 

 a nauplius hatched from every egg in the sac. In the other cases 

 a majority of the eggs hatched. Two sacs which were known to 

 have been centrifuged before the nuclear walls had broken down 

 also gave rise to normal embryos. The cleavage planes and hence 

 the spindles in the eggs of a normal sac, as already pointed out, 

 bear no constant relation to each other, though they are to a cer- 

 tain degree determined by pressure conditions in the sac. Cen- 

 trifugal force acting upon the sac, therefore, acts upon the spindle 

 of each egg from a more or less different angle. Moreover, in 

 each of the four egg sacs described above the layers bore a dif- 

 ferent relation to the axis of the sac. These considerations, 

 together with the fact that even in normal sacs all the eggs do not 

 develop, make it probable, I think, that the spindle is never per- 

 manently injured by centrifuging. 



The relation of the nucleus and of cleavage spindles to the 

 layers is the same in eggs centrifuged after, as in those centri- 

 fuged before cleavage. When the stratification is either parallel 

 or perpendicular to the cleavage the long axis of the spindle is 

 always parallel to the layers and also to the first cleavage plane. 

 When the stratification is oblique to the cleavage the spindles 

 conform to the stratification and make an acute angle with the 

 first cleavage plane (fig. 5). Moreover, in the oblique eggs the 

 spindles of the two blastomeres lie sometimes at right angles to 

 each other, whereas normally they are parallel (fig. 4). 



The condition of the spindle after centrifuging varies and the 

 variation affords one of the most striking evidences of spindle 

 movement. The sectioned sac shown in fig. 3 illustrates the dif- 

 ferent conditions. Egg (a) represents the same condition as that 

 found in the sea urchin. The spindle lies close under the white 

 cap and its parts retain their normal relation to each other. In 

 (b) and (c) the equatorial portion of the spindle lies close under 

 the cap but the poles are bent far down toward the yolk layer. 



That the distortion of these spindles is not to be explained by 

 differences in specific gravity of its parts is shown by the straight 

 spindle in (a). It is rather due to the shape of the cell. Egg (a), 



