Abstract
A combined study using the light and electron microscopes was undertaken on the oocytes, vitelline cells, oviduct, vitelline collecting duct, vitelline reservoir, vitelloduct, ovi-vitelloduct, ootype, Mehlis' gland, and uterine regions. Cortical granules were not observed beneath the plasmalemma of the oocyte. The general structure of the vitelline cell corresponds to that found in other digeneans. The results indicate that in A. simplex fertilization occurs in the distal part of the oviduct. The path of the vitelline cells and the oocyte towards their point of meeting is described and discussed. It is suggested, as a working hypothesis for further studies, that the combined lengths of the vitelloduct and the ovi-vitelloduct in this species is a regulatory factor for the determination of the number of vitelline cells in the egg. A possible course of events for this process is presented. Three types of cells, two of which are secretory, are present in the region of the Mehlis' gland. Two types of secretory products are found in the long cytoplasmic processes which open into the ootype. One of these types is discharged only into the proximal ootype, while the other is discharged into the remainder of the ootype. Previous ultrastructural reports on similar secretory products on other digeneans are reviewed and discussed, as is the terminology of these secretory products. The related terms DB- and MB-secretory products are proposed for the two types of secretion. Both types of secretory product drastically change their characteristics upon entering the lumen of the ootype where, nevertheless, it was possible to identify both types of secretion. It is suggested that the third and interstitial non-secretory type of cell present in the region of the Mehlis' gland has a ‘skeletal’ function; a function which is also suggested for the microtubules of the cytoplasmic processes (unicellular ducts) projecting from the secretory cells. The earlier proposed sensory function of the surface bulbs around the genital atrium is confirmed and their characteristics indicate that they function as tango-receptors for use in copulation. A third type of tegumental spine, the genital spine, is described and discussed.
