Local accumulation of Feulgen-positive granules in the egg cortex of Dentalium dentale L.

A special feature of eggs of several species of annelids and molluscs is the formation of a polar lobe at stage-specific intervals during the cleavage phase. In the scaphopod Dentalium, a polar lobe develops at the first, the second and the third cleavage (Wilson, 1904). The cytoplasm set apart in the polar lobes is ultimately confined to the D blastomere and its derivative cells. Wilson observed that after removal of the polar lobe at the first cleavage a larva develops which lacks an apical tuft and the post-trochal region. After removal of the polar lobe at the second cleavage a larva develops which lacks most of the post-trochal region but possesses an apical tuft. A similar result to the one following the removal of the first polar lobe is obtained by removal of the vegetal one-third of an unfertilized egg. These results indicate that morphogenetic determinants present in the first polar lobe are already set aside in the uncleaved and unfertilized eggs, and that the determinants for the apical tuft are no longer present in the second polar lobe. The study of maturation in amphibian oocytes has shown that a relationship exists between nuclear DNA and Feulgen-staining granules appearing in the cortex after breakdown of the germinal vesicle (Brächet 1965, 1967). As Brächet pointed out this cortical localization may be significant with respect to the importance of the dorsal cortex (grey crescent) in later development. Therefore, we have studied the distribution of DNA in the egg cells of Dentalium as far as it can be traced with the Feulgen method.