The vitellogenesis in Bothriocephalus clavibothrium was studied by means of histochemistry and electron microscopy methods.

In the course of vitelline cell cytomorphosis, three following stages have been recognized: 1) immature, 2) maturing and 3) mature.

The immature vitelline cell represents the gonial type.

The maturing one shows extended granular endoplasmic reticulum, Golgi complex and several mitochondria. Maturation involves an increase in size of two or three times and formation of numerous lipid droplets, heterogenous shell globule vesicles and large amounts of glycogen (mainly in rosettes = α glycogen). The protein precursor of shell glubules seems to be concentrated within minute Golgi vesicles which migrate toward the larger vesicles and become fused together. The fusion of larger heterogenous vesicles containing several shell globules was also observed.

Mature vitelline cells leaving the vitelline gland show desintegration of endoplasmic reticulum and their cytoplasm becomes completely filled with shell globules, glycogen and lipid droplets. The rôle of those products synthesized and accumulated by vitelline cells is discussed. The heterogenous shell globule vesicles are considered as a shell precursor; the glycogen as a food store for the developing embryo. The rôle of lipid droplets remains uncertain.