CYTOLOGICAL STUDIES OF CEXTRIFUCxED EGGS 
621 
Orientation of the eggs in the centrifuge 
When the eggs are centrifuged before the germinal vesicle 
breaks down, the latter is driven to one pole; if centrifuged not 
too hard, the yolk ring forms ; if hard, the yolk passes more directly 
to the outer hemisphere. If these eggs are kept until the time 
when the polar spindle forms and then killed and preserved it 
will be found that the polar spindle reaches the surface in position 
to give off the polar body only in the lighter hemisphere, never in 
the yolk hemisphere. This may mean either that the eggs orient 
so that the pole is towards the center and the antipole outward ; 
or that only those spindles can reach the surface that are in an 
egg whose pole is in the lighter hemisphere; while those in eggs 
with their pole in the yolk hemisphere fail to penetrate the yolk. 
The second conclusion is, I believe, the correct one. In support of 
this view is the fact that in many of the eggs polar spindles can be 
found at this time in the interior of the egg, and some of these con- 
tain the chromatin in the form of two separate plates near each 
pole of the spindle. Such a condition is found in the normal 
spindle only when the polar body is ready to be given off. Lillie 
has found a similar condition in Chaetopterus and interprets it 
in the same way. 
If the eggs are centrifuged after the polar spindle is formed the 
same state of affairs is found; spindles are found in the light hemi- 
sphere at the surface and inside the egg; none in the yolk hemi- 
sphere. In this case those spindles that lay in the yolk hemisphere 
must have been driven into the interior of the egg. 
Eggs that have given off the polar bodies have been examined, 
but so far without much success, since it has been difficult to 
detect the polar body with certainty after centrifuging, but the 
evidence so far as it goes indicates that polar bodies may lie at 
any point on the surface of the egg, and this condition is opposed 
to the idea of orientation on the machine. 
In Cerebratulus there is, for a time, a long or short stalk of 
protoplasm at the antipole of the egg. I have tried to determine 
the method of falling of the egg by this means. Since the stalk 
lies within the jelly membrane it probably has no directive influ- 
