The process whereby maternally or zygotically synthesized molecules become situated in specific spatial locations in the egg or zygote. This has been most widely examined in the Drosophila egg (e.g., in formation of the pole plasm (q.v.) or in positioning of cytoplasmic determinants (q.v.) that are later required for embryonic body pattern formation) and thought to be a stepwise process involving synthesis of the maternal product, its transport to the desired location, anchoring, and maintenance of localization. These steps are dependent upon sequential gene expression, cytoskeletal elements, and cell organelles. See Balbiani body, bicoid, maternal effect gene, maternal polarity mutants, mitochondrial cloud, sponge body.
Subjects: Genetics and Genomics.