Localization of morphogenetic determinants in a special cytoplasm present in the polar lobe ofBithynia tentaculata (Gastropoda).

In the first polar lobe ofBithynia eggs a special plasm, the vegetal body, is present. It consists of a cupshaped aggregate of small vesicles. Centrifugation of eggs prior to first cleavage may result in displacement of the vegetal body. In about 50% of thecentriguged eggs the vegetal body is found outside the polar lobe, in one of the blastomeres. Removal of the polar lobe from non-centrifuged eggs always leads to severe defects in development. When the lobe is removed from centrifuged eggs, however, about 50% of the eggs develop into normal embryos. It is concluded that the presence of the vegetal body in a blastomere suffices to ensure normal development and, hence, that the polar lobe-specific morphogenetic determinants are contained within the vegetal body.