Differentiation of the vitelline coat in the ascidianCiona intestinalis: an ultrastructural study.

We have studied the differentiation of the vitelline coat (VC) of the ascidianCiona intestinalis. In the young previtellogenic oocyte the vitelline coat precursor material (VCPM) makes its first appearance as patches of fibrous material in close apposition to the outer surface of the oocyte. The presence of subcortical vescicles containing a fuzzy electron-dense material and their opening into the oocyte surface parallels the formation of VCPM.