Role of the Vegetal Body in the Regulation of Development inBithynia tentaculata(Prosobranchia, Gastropoda)

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Abstract
In the first polar lobe of the egg of Bithyma tentaculata there is a cup-shaped mass of small vesicles rich in RNA, called the vegetal body. It is passed to the CD blastomere with the polar lobe and then its contents are distributed to C and D. When the polar lobe is removed, embryos form neither mesentoblast nor mesoderm bands. They fail to establish bilateral symmetry or to form eyes, foot, operculum and shell. AB half embryos develop only the structures found in lobeless embryos, while CD halves develop lobe dependent adult structures such as eyes, foot and shell. AD- and BC-2/4-embryos develop similarly and both have lobe dependent structures. Three quarter embryos develop as in other molluscs except that ABD (−C) develops poorly and ABC (−D) develops all lobe-dependent structures. Thus, all evidence available indicates that C receives cytoplasm usually restricted to D in molluscs and therefore has the capacity to exert a similar morphogenetic influence on development.

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