Microscope observations of rabbit zygotes demonstrate that a sperm aster forms in association with the male pronucleus approximately 1 h postinsemination and consists of two regions. One, the centrosphere, contains a dense aggregation of cisternae of smooth endoplasmic reticulum and microtubules. The second consists of fascicles of microtubules which emanate from the centrosphere. Fertilized rabbit eggs were cultured in medium containing colcemid in order to determine its effects on various events of fertilization, such as movements of the male and female pronuclei and DNA synthesis. No evidence was obtained to indicate that a sperm aster is formed in colcemid-treated zygotes. In addition, migration and close apposition of the pronuclei do not take place. Breakdown of the pronuclear envelopes and condensation of the maternally and paternally derived chromosomes occur even though the pronuclei fail to migrate centrad. Autoradiographic analysis of the synthesis of DNA by both pronuclei demonstrates that their migration into close apposition to one another is not required for the incorporation of tritiated thymidine.

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