Preblastoderm Nuclear Division in the Embryo of the Large Milkweed Bug, Oncopeltus fasciatus (Dallas)

The preblastoderm nuclei of the large milkweed bug, Oncopeltus fasciatus (Dallas), divide in the absence of centrioles and a spindle fibre apparatus. The preblastoderm nuclei occur in cytoplasmic islands that are not bound by cytoplasmic membranes. The chromatin condenses in association with the nuclear envelope and intranuclear lamellae into two compact masses. The chromatin of the sister nuclei disperse as the nuclei increase in volume and move away from each other in a common cytoplasm. A model is proposed for the orderly separation of the chromatin of the indicated eukaryotic system that is patterned after the site and site territory concept for the control of prokaryotic DNA replication by the membrane. In the case of the milkweed bug preblastoderm nuclei, the nuclear envelope would play a distributive role by providing only one site and site territory in each half of the nucleus for the attachment of a homologue. The presence of a site on the membrane for the attachment of a given chromosome would prevent the formation of a new site for the chromosome homologue within a certain distance.