The Ultra Structure of the Vitelline Cells of Haematoloechus

Abstract

For a number of years investigators generally agreed that in oviparous trematodes the vitelline cells provided the food for the while the shellor Mehlis' gland provided the shell. Although the nutritive role of the vitellaria has remained unquestioned, serious doubts as to the capacity of the shell gland to produce all of the materials necessary for the construction of the shells have arisen from time to time. Probably the small size of the shell gland in most trematodes or its absence in others led investigators to seek elsewhere for the source of at least part of the shell material. During the early 1930's a number of studies reported an additional role for the vitelline glands, that of providing material for the shell. Perhaps the statement by Yosufzai (1953) that the egg shell is formed by a hyaline secretion of the gland (Mehlis) and (is) temporarily reinforced by vitelline granules which serve as nutriment best reflects the present day thinking with regard to the function of vitelline cells in the majority of trematodes. There is less agreement as to the specific region of the body in which the vitelline secretions released from their cells of origin. Sommer (1880) reported that the vitelline products freed long before any vitelline reservoir is Stephanson (1947) described the release of vitelline substances as the result of mechanical churning in the uterus in the presence of sperm, which implies that the intact cells pass from the vitellaria into reproductive system via the vitelline ducts. Willmott (1950) contributed the suggestion that the vitelline cells give up the drops of shell-forming substance in the presence of oocytes in the central chamber (ootype) of Mehlis' gland. Yosufzai (1953) believed that the release of granules takes place close to the opening of the oviduct. Dawes (1956) in his study on Hexostoma describes (and illustrates) the intact vitelline cells passing along the vitelline duct and he notes that they are crowded with droplets of secretion which eventually cause the cell membrane to bulge and in process of being extruded when the ootype is reached. Some observations on the release of vitelline substances in Haematoloechus reported here which appear to confirm the 1880 findings of Sommer.