ABSTRACT
A study has been made of the rate of fall of the nucleolus through the nuclear fluid of echinoderm oocytes. Analysis of the results obtained on the oocytes of Echinus esculentus leads to the following conclusions :
The absence of sudden velocity changes during the fall indicates the absence of gross heterogeneities in the nuclear fluid.
The nuclear fluid is thixotropic. Complete reconstitution of its elastic structure takes place within 10 min. of its disruption by the moving nucleolus.
In addition to the gravitational force producing the fall, there is acting on the nucleolus a much weaker force, constant in magnitude and direction for any one oocyte.
The presence of this additional force is indicated by a constantly reproducible difference in the rate of nucleolar fall in two opposing directions, and also by horizontal motion of the nucleolus during its fall.
The force does not arise from viscosity differences in different regions of the nucleus ; the nuclear fluid is apparently homogeneous and has an absolute viscosity of approximately 0·1, i.e. 10 times that of water. Neither nucleolar asymmetry nor streaming of the nuclear protoplasm can account for the phenomenon.
The electrical nature of the force on the nucleolus is suggested by the elimination of these possible mechanical explanations and by the presence of a surface charge on the nucleolus. The order of magnitude of the potential field required by this hypothesis is consistent with the energy output of the respiring oocyte. The polarized force on the nucleolus is abolished in M/500 potassium cyanide, and also by heating above 30° C.
The bearing of these results on the structure of the nucleus and on the origin of polarity in the unfertilized egg is discussed.
