In the female reproductive system there are seperate organs for the production of the ovum, and for the production of the yolk cells and shell material. An organ called the öotype ( "Oo") receives eggs (as many as 25 000 a day in some trematodes) from a single ovary, which is may vary in shape and position depending on the trematode species, ("O"). Surrounding the öotype is a glandular region, the Mehlis gland ("M"). The function of this gland is unclear, but may help nourish the developing egg, and provide lubricating fluid, aiding the movement of the eggs in the öotype and uterus. Sperm may enter the female reproductive system either via a specialised structure, "Laurer's" canal ("l"), opening from the dorsal surface of the worm, or more commonly via the common genital atrium, (into which the uterus, ("Ut") open), either during copulation, or during self-fertilisation. The ova released from the ovary reaches the öotype via the oviduct, where it is fertilised by the sperm, and is then surrounded by yolk cells and material that hardens around the egg ("E"), to form an egg shell. Both the yolk cells and egg shell material are derived from the vitelline glands ("vt"), which are connected to the öotype via the vitelline duct, ("vt.d"), often entering a small vitelline reservoir first. The fertilised eggs then enter the uterus. This is usually a highly coiled structure, the terminal part of which (the metraterm) is often equiped with a specialised muscular wall. The eggs then leave the parasite as they pass out of the genital pore, to continue the parasites lifecycle.