Fine structural studies of the bipolarization of the mitotic apparatus in the fertilized sea urchin egg. II. Bipolarization before the first mitosis.

After fusion of the two pronuclei the former sperm head centrosome is attached to the envelope of the zygote nucleus while the former mitochondrial centrosome is only loosely associated with it. These two centrosomes are not yet in opposite positions but are separated from each other by spreading centrosomal material. This spreading is mediated by microtubules. It is concluded that the attached centrosome remains stationary while the motile one is moved around the nuclear surface to an antipodal position, 180 degrees from the other. The first bipolarization process which occurs prior to the breakdown of the nuclear envelope is compared to the second and all other bipolarizations: Similarities and dissimilarities can be found, but similar or identical mechanisms for both processes are assumed.