The structure of a morphogenetic cytoplasm, present in the polar lobe of Bithynia tentaculata (Gastropoda, Prosobranchia).

In the first polar lobe of the egg of Bithynia tentaculata a cup-shaped mass of small vesicles is described, which fills the greater part of the lobe. It is named the 'vegetal body'. With methyl green-pyronin the vegetal body stains clearly, but after treatment with RNase no staining occurs, thus indicating the presence of RNA. The first polar lobe of Bithynia is of great importance for further development of the embryo and it is argued that the vegetal body could be a morphogenetic cytoplasm, responsible for the developmental effects of the polar lobe.