ULTRASTRUCTURAL STUDIES ON THE NEMATODE XIPHINEMA-DIVERSICAUDATUM - EGG-SHELL FORMATION

The ultrastructure of the formation of the egg shell in the longidorid nematode Xiphinema diversicaudatum is described. Upon fertilization a vitelline membrane, which constitutes the vitelline layer of the egg shell, is formed. The chitinous layer is secreted in the perivitelline space, between the vitelline layer and the egg cell membrane. On comp letion of the chitinous layer, the material of the lipid layer is extr uded from the egg cytoplasm to the outer surface, through finger-like projections. Both chitinous and lipid layers are secreted by granules in the egg cytoplasm that disappear as the layers are completed. Chiti nous and lipid layers are formed during the passage of the egg through the oviduct. The vitelline layer is enriched with secretions produced by the oviduct cells and then by phospholipids secreted by the cells of the pars dilatata oviductus. The inner uterine layer is also formed by deposition of secretory products apposed on the egg shell in the d istal uterine region and Z-differentiation. In the proximal part of th e uterus, the egg has a discontinuous electron-dense layer, the extern al uterine layer. Tangential sections between chitinous and uterine la yers revealed the presence of holes, possibly egg pores, delimited by the two uterine layers.