CYTOLOGICAL STUDIES OF CENTRIFUGED EGGS 629 
the cytoplasm, the aster retains its form showing the relative 
rigidity at this time. Incidentally the results indicate as pointed 
out, that the movement of the aster in the egg may be purely 
passive, and not be due, as has been suggested, to its re-formation 
at each step in its process. 
Eggs centrifuged during the pronuclear period, and returned to 
sea water, show a spindle near the center of the egg that resembles 
the normal segmentation spindle. In this case as in Cumingia 
there is a return of the chromatin, and of the asters to the interior 
of the egg. 
The stages that furnish the most instructive results are those 
during the period of formation of the segmentation spindle and 
just before the cytoplasmic division when the chromosomes have 
separated and begun to form separate vesicles. When eggs with 
a segmentation spindle are strongly centrifuged they may be 
driven out into bottle-shaped forms or into strings. The spindle 
lies just above the yolk-ball, where the polar half normally 
constricts from the antipolar hemisphere. Consequently this 
spindle becomes caught in the elongation and may suffer distor- 
tion. Such a case is shown in figs. 41-43, where the fibres of the 
poles are thrown into curves or spirals, and the central spindle 
bent in its middle between the plates of chromosomes, fig. 42. This 
figure shows that the rays are not broken but bent and their 
semisolid nature made evident. The ^'neck" is narrower than the 
actual size of the segmentation spindle, hence in order to be con- 
tained in this space the long fibres are bent. It should be recalled 
that at this stage the astral system has reached its maximum, and 
the rays fill almost if not quite the entire egg. 
When the chromosomes reach the poles they become individu- 
ally vesicular and later these fuse into a single nucleus near each 
astrosphere. At this time just prior to the cytoplasmic division 
the effect of the centrifuge is to carry the vesicles towards the 
pole, and since these are closely connected with the astral system 
that system is dragged towards the pole. The results produce 
effects like those show^n in plate 8, figs. 51, 53. These stages show 
the group of vesicles or the two nuclei, if the vesicles have fused, 
beneath the polar cap. From them streamers representing the 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 9, NO. 3. 
