Abstract
It is known that the class Trematoda includes exclusively parasitic forms of flatworms. In this regard, they have formed a variety of morphological and functional adaptations to a parasitic lifestyle. One of them is the colossal fecundity, which provides the possibility of rapid and wide settlement and detection of hosts. Therefore, the study of the sexual reproduction of helminths is the central problem of parasitology. The study of the functional morphology of the reproductive system of trematodes in general and its parts is of particular interest in solving such an important problem in theoretical and practical terms as the directed regulation of the fecundity of end parasites. The present article provides information on the functional morphology of the vitelline cells of the Azygia lucii trematode, describing in detail the micromorphology and ultrastructure of the vitelline cells and the development stages of vitelline cells in the course of vitellogenesis. As a result, the knowledge gained on the peculiarities of the structure and maturation of Azygia lucii vitelline s will significantly supplement the knowledge about the peculiarities of the structure of the female reproductive system of Strigeidae trematodes and the fecundity of end parasites.