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Ultrastructural descriptions of the oviduct, fertilization canal, seminal receptacle, ovovitelline duct, vitelline reservoir, ootype, Mehlis' gland, proximal uterus, and neurosecretory elements associated with egg-forming ducts are given for the progenetic spathebothriidean tapeworm, Diplocotyle olrikii from the body cavity of Gammarus oceanicus. The functional significance of cortical granules of the oocyte, as necessary elements for joining vitelline material to an oocyte in the ovovitelline duct, is established. The proximal ootype has a vesicular epithelium and is the site of initial, nascent eggshell formation. Precursors of nascent eggshell are vesicles, synthesized in both the proximal ootype wall and vitelline cytoplasm that become associated with the newly formed shell. Major shell structure comes from subsequent deposition of shell globules from a disintegration of vitelline clusters. Mehlis' gland has a single secretory cell type. Secretory granules from Mehlis' gland become associated with the developing egg that passes through to the distal ootype and proximal uterus where egg-formation is completed. It is not known, however, whether Mehlis' gland secretion promotes breakdown of free vitelline cells, liberation of shell globules, confluence of shell globules on the developing eggshell or provides further structural components for the shell. Despite some differences in ootype morphology, the basic process of eggshell formation in D. olrikii may share much in common with the Pseudophyllidea and Caryophyllidea. Small vesicles and dense-core vesicles are in nerve terminals near duct musculature. Nerve terminals with large dense vesicles are described near, in, and within the seminal receptacle, fertilization canal and distal ootype. The possible physiological effects of exocrine neurosecretions are discussed.
The ultrastructure of the ovary and oogenesis are described from the immature and sexually mature female reproductive system of the progenetic spathebothriidean tapeworm, Diplocotyle olrikii from the body cavity of Gammarus oceanicus. Two types of cells are described: germinal (oogonia, oocytes) and interstitial. A comparison is made of the fine structure of oogonia, early and advanced maturing oocytes and mature oocytes. Two types of inclusions, cortical granules and lipid droplets, are produced by maturing oocytes, and remain in the cytoplasm of mature oocytes within the ovovitelline duct lumen while only lipid droplets are evident in the oocyte cytoplasm of intrauterine eggs. The fate and possible functions of both inclusions are discussed. The interstitial component of the ovary is a syncytium. The maturing oocyte surface is prolonged into lamellae, forming a lamellar mesh with adjacent germ cells and close association of interstitial mitochondria. Deep invaginations of the ovarian basement layer between numerous folds of ovarian lobules facilitate close contact of the interstitium and sarcoplasmic glycogen-rich processes with maturing oocytes. Synchronism in maturity among all of the oocytes in the ovary is shown at different stages of oogenesis. Such a pattern of oogenesis results in the production of many eggs at the same stage of development and is considered an adaptation for the dissemination of fertilized eggs that occurs only at the death of the gammarid host.
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