ULTRASTRUCTURAL AND CYTOCHEMICAL STUDIES ON THE GERMANIUM OF DICROCOELIUM-DENDRITICUM (PLATHELMINTHES, DIGENEA)

The unpaired germarium of Dicrocoelium dendriticum contains many femal e germ cells at different stages of maturation and is enveloped by a f ibrous basal lamina-like structure and a multilayered cytoplasmic shea th whose origins and functions are discussed. The maturation process o f primary oocytes occurs completely within the prophase of the first m eiotic division. It has been divided into three stages, as previously suggested for monogeneans. Stage I corresponds to oogonia and early oo cytes which are located in the distal germinative area of the gonad. T hese cells are characterized by a high nucleo/cytoplasmic ratio and a poorly differentiated cytoplasm. Stage II corresponds to maturing oocy tes grouped in the central area of the gonad and exhibiting long synap tonemal complexes and a prominent nucleolus. The main feature of cytop lasmic differentiation is the increase in the number of RER and Golgi complex which are involved in the production of small electron-dense g ranules. Stage III corresponds to mature oocytes located in the proxim al area of the germarium near the origin of the oviduct. In this stage , the granules become regularly distributed in a monolayer in the peri pheral ooplasm and make contact with the oolemma. They show a distinct ive complex structure, are composed of proteins and glycoproteins and do not contain polyphenols. Their possible role is discussed in relati on to chemical composition and previous studies on other Plathelminthe s. Neither yolk globules nor glycogen are present in the oocytes.