442 
EDWIN G. CONKLIN 
this stage orients so that the animal pole is directed toward the 
center of the centrifuge, or if the eggs do not orient on the cen- 
trifuge, the maturation spindles must be shifted so that they come 
to lie in the axis of stratification. I have not been able to de- 
termine with certainty which of these possibilities is actually 
realized in these eggs, but in Crepidula, as I shall show in another 
paper, it is almost impossible to move the spindles by centri- 
fugal force; it seems probable therefore that in this case also the 
spindles do not move, but that the eggs orient, so that their 
chief axes come to lie in the axis of stratification. 
After both maturation divisions the stratification may take 
place in any axis; consequently the polar bodies may lie on any 
zone. However in all cases the egg retains or recovers its original 
polarity after its removal from the centrifuge. I conclude there- 
fore that the chief axis of the egg is fixed at all stages and may not 
be altered by the shifting of egg substances, or maturation spin- 
dles, or the point of extrusion of polar bodies. This conclusion 
is in substantial agreement with the work of Lillie (1909) and 
Morgan (1909). 
4. As the position of the maturation spindle under normal 
conditions proclaims the polar differentiation and the chief axis 
of the egg, so the position of the first cleavage spindle in normal 
eggs announces the plane of bilateral s^^mmetry. This spindle 
lies in the plane of symmetry and the first cleavage separates 
an anterior from a posterior blastomere. The ooplasmic sub- 
stances may be shifted in any direction with regard to this plane of 
symmetry, usually without changing the plane itself, and since 
this may occur before the cleavage spindle is formed it is evident 
that the position of the spindle is the result rather than the cause 
of bilaterality. The same is true of the positions of the egg and 
sperm nuclei and of the cleavage centrosomes. 
In some cases the cleavage spindle may be moved in position 
so that the first two blastomeres are unequal in size (fig. 32) and 
yet normal development may result. There is little evidence 
iowever that the cleavage spindle can be turned from its position 
at right angles to the chief axis, or that the cleavage furrow can be 
made to form an angle with that chief axis. But while the nucleus 
