Vitelline cells, which are added to the fertilized ovum when eggs are formed in Platyhelminthes, are known to produce and secrete proteins containing vesicles for construction of the eggshell. In this study, another particular type of carbohydrate-containing vesicle is described, in the vitellocytes of Digenea. These vesicles play a role in the hatching of the miracidium from the egg. Cytochemical analysis and the binding of lectins with specificities for a variety of sugar residues revealed that the vesicles contain neutral, glycogen-like polysaccharides composed of glucosyl/mannosyl residues. The vesicles are produced at a late maturation stage of the vitellarium cells. Vitellocytes enclosed in the eggs provide nutrients for the embryo, but retain the glycan vesicles until late embryogensis. Then the vesicles merge and swell to the dimensions of the vitellocytes, and these coalesce into two vacuoles that fill the space between the embryo and the eggshell. Hatching of the miracidium is induced by exposure to light and a slight rise in temperature, i.e. conditions found in the natural environment in the morning. The eggs' internal hydrostatic pressure rises, probably due to a depolymerization of the polysaccharides that causes an osmotically driven water influx. Finally, the operculum of the egg bursts open and the miracidium escapes.