The immature vitelline cells of D. latum have a large nucleus and cytoplasm
with high content of free ribosomes and mitochondria. They
undergo maturation, which is characterized by increase of cytoplasm
volume and the appearance of extensive Golgi complexes and the granular
endoplasmic reticulum (GER). The amounts of these organelles increase
greatly in the early stage of maturation of the vitellocytes. These
organelles are responsible for the secretion and packaging of shell globules
and the formation of membrane-bound shell globule clusters,
which play a key role in the egg-shell formation.