Yolk granules. Coloured scanning electron micrograph (SEM) of yolk granules (green) in an extract of cytoplasm taken from a frog (Xenopus sp.) oocyte (egg). Yolk granules are storage bodies for a type of lipoprotein called vitellin, which nourishes the embryo during the first stages of development. The blue structures are endoplasmic reticulum (ER) vesicles. The ER is a membrane-bound cell organelle that is the site of lipid synthesis and the production of membrane- bound proteins.