The egg cortex can be defined as the periferal region of the egg strongly adhering to the plasma membrane . It can withstand low centrifugal forces that stratify egg components. Isolated cortices can be prepared by gently opening eggs or shearing eggs attached to surfaces with milieu which resembles the internal cytoplasmic milieu. In this case fields of cortices are obtained as shown on the left images (see methods).
In many species (ascidians,echinoderms, mammals...) there are 3 main components of the scaffold which constitutes the isolated cortices.
--The plasma membrane and vitelline membrane
-- A network of microfilaments adhering to the plasma membrane and its microvilli
-- A network of cortical Endoplasmic Reticulum (ER) adhering to the plasma membrane by feet
In addition various cortical vesicles, microtubules and intermediate filaments are isolated with the cortex depending on the stage of oogenesis and development.
In ascidians the cortical ER and microfilament networks are organized in a gradient along the animal-vegetal axis. The networks are reorganized after fertilization and during the first cleavages. We have recently shown that at least one mRNA determinant (macho 1 the determinant for muscle) is attached to the cortical ER and translocates with cortical ER after fertilization.

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