Local surface differentiations at the vegetal pole of the eggs of Nassarius reticulatus,Buccinum undatum,and Crepidula fornicata (Gastropoda,Prosobranchia)

In the eggs of the gastropods Nassarius reticulatus, Buccinum undatum, and Crepidula fornicata, local surface differentiations are described, using transmission and scanning electron microscopy. The differentiations consist of special microvilli (Nassarius), ridges composed of microvilli (Buccinum), or extensive surface folds (Crepidula). They occur on the polar lobes of these eggs and it is argued that a relation may exist between the surface structures and the localization or expression of the morphogenetic factors present in the polar lobes. The presence of the surface differentiations before the appearance of the polar lobe and their persistence after resorption of the lobes are shown in Nassarius and Crepidula eggs. Evidence is presented that the differentiated area at the vegetal pole of the Nassarius eggs corresponds to the former contact area of the oocyte with the follicle cells in the ovary. This correspondence is considered as an argument in favor of the theory that the egg cortex may locally acquire special properties under the influence of the surrounding structures in the ovary.