CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 365 



Spindle grows longer during the anaphase. If the maturation 

 spindles are prevented from shortening, giant polar bodies are 

 formed since the division plane must pass through the equator 

 of the spindle. If the first cleavage spindle is turned into the 

 chief axis of the egg the cleavage plane is equatorial, instead of 

 meridional as it should be, since the division plane must be at 

 right angles to the spindle axis. 



However the initial position and direction of the spindle may 

 differ from its definitive orientation. In several earlier papers 

 on Crepidula ('97, '98, '99, '02) I have shown that the spindle 

 may form out of its definitive position and subsequently be moved 

 into it by the activity of the cytoplasm. Lobes of cytoplasm 

 may indicate where micromeres will form while the newly formed 

 spindles are yet some distance away from these lobes ; ultimately 

 one end of a spindle moves into each of these lobes and then the 

 cell division takes place through the equator of the spindle 

 cutting off a micromere from a macromere. 



Without doubt the position and direction of a cleavage furrow 

 is determined by the position and direction of the fully devel- 

 oped spindle, but what determines the orientation of the latter? 

 It is sometimes assumed that the orientation of a spindle is de- 

 termined by yolk or other inclusions, for example that micro- 

 meres are formed at the animal pole because yolk is segregated 

 at the vegetal pole and this displaces cytoplasm, nuclei and 

 spindles toward the animal pole, — but this is quite erroneous. 

 In this paper as well as in a former one ('12) I have shown that 

 the pattern of cleavage is more or less independent of the amount 

 of yolk, oil, water or other inclusions contained in a particular 

 egg or blastomere, and Lillie ('06) and Morgan ('10) found this 

 to be true in the eggs which they centrifuged. If the yolk which 

 collects at the centrifugal pole or the oil which collects at the 

 centripetal pole are thrown out of the egg completely the re- 

 mainder of the egg which contains the nucleus and the material 

 of the middle zone may segment like a normal egg, the first and 

 second cleavages being approximately equal and the subsequent 

 ones unequal, thus giving rise to four macromeres and to three sets 

 of micromeres which form from these. If the volk is thrown 



