Stages in the assembly of the egg in the monogenean skin parasite Entobdella soleae have been studied using a fast preservation technique for transmission electron microscopy. The first event is the release by the germarium of a fertilised oocyte, which travels to the ootype followed by many vitelline cells. There are two types of Mehlis' gland and the secretion from one of these (beta) is thought to promote the release of the vitelline droplets, which fuse peripherally in the distal tetrahedral chamber of the ootype to produce the eggshell. Initially, the zygote lodges in the distal corner of the chamber, perhaps held in place by cortical granule material, and prevents shell deposition in this corner. However, this is temporary, and when the zygote leaves the corner the opercular eggshell is laid down. The egg appendage is assembled in the proximal tubular part of the ootype and the adhesive droplets on the appendage are derived from the second (alpha) type of Mehlis' secretion.
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