CYTOLOGICAL STUDIES OF CEXTRIFUGED EGGS 
603 
centrifuged egg gives the appearance seen in plate 1, figs. B-F 
where the pigment and the oil field are flattened and sharply 
separated from the protoplasm. 
A section of an egg centrifuged at this time is shown in plate 
2, fig. X. The yolk is all collected at one pole; the pigment has 
been dissolved ; the position of the oil is shown by the vacuolated 
protoplasm at the pole opposite to the yolk. The relatively large 
band of protoplasm contains the spindle. The spindle may lie 
at any angle in regard to the plane of stratification. The egg at 
this time does not orient as it falls in the centrifuge tube. Hence 
the oil and the yolk may be in an}^ part of the egg in regard to its 
poles, as is shown by the formation of the polar bodies that are 
given off at the original pole of the egg, irrespective of the materials 
that have been thrown into that region as seen in figs. O, P, S. 
When the egg is centrifuged at once, and then fertilized, the 
polar body is given off in less than half an hour. As the spindle 
moves to the surface it displaces there whatever material has been 
driven to the animal pole. A clear polar field again appears with 
the polar body in its middle (plate 1 fig. B-F). If the pigment and 
yolk are displaced, the egg appears at this time as shown in figs. B, 
D. The pigment (or yolk) is pushed away from the pole and as- 
sumes the form of a ring. If the spindle pushes through the oil 
ring, a clear region appears, fig. C, which displacing the oil forces 
it around the sides. If the spindle appears at the side of the clear 
area (figs. E, F) this region too becomes clearer than before. 
Under these circumstances the oil and the pigment are often 
carried around the opposite sides of the egg until they touch each 
other (fig. F). The movements of the pigment and oil fields show 
clearly that extensive shifting of the contents of the egg takes 
place at the time when the polar bodies are formed, and it is 
apparent that the movements are the result of materials in the 
interior of the egg moving up to the surface. This migration is 
connected with a change in position of the karyokinetic spindle. 
Sections through the egg (fig. P, Q) confirm what can be observed 
in the living egg. I have found few cases in which the spindle 
moves through the middle of the oil field. This may mean that 
there is some mechanical difficulty in the way of such movement, 
