[261] _Monogenea_: Trematoda in which the anterior sucker, if present, is double. Development without an intermediate host. [Illustration: FIG. 128.--Ovum of _Fasciola hepatica_, L., cut longitudinally. The lid has been lifted in the process. Within the egg are numerous yolk cells, and at the lid end there is the still unsegmented ovum (dark). 240/1.] [Illustration: FIG. 129.--Miracidium of _Fasciola hepatica_ that has just hatched from the egg, with a distinct cuticular ciliated epithelium. Magnified. (From Leuckart.)] (2) _Formation of the Ova._--The ovarian cells arising from the ovary first become mature after their entry into the oötype by the formation of three polar bodies, fertilization then taking place. At the same time as the ovarian cell a number of yolk cells from the vitellarium and secretion, drop by drop, from the shell gland reach the oötype.[262] The shell is then formed during the generally active contractions of the oötype walls and then passes on into the uterus. In the uterus of the endoparasitic trematodes the eggs accumulate more and more, often in large quantities, while in ectoparasitic species generally only one or some few eggs can be found. The completed ova are of various forms and sizes. They are mostly oval, at all events in the digenetic trematodes, and the yellowish or brown shell is provided with an opening at one pole which is closed by a watch-glass-shaped lid (operculum). Appendages (filaments) on the shell--at one or both poles--are uncommon, but are the rule in the ova of the _Monogenea_ (ectoparasitic species). [262] [Recent work (_e.g._, Goldschmidt, _Zool. Anzeiger_, xxxiv, p. 482) has shown that the older views regarding the formation of the egg must be modified. In certain species, at any rate, the shell material is formed by the yellow droplets of the yolk glands and not by the so-called shell gland (Mehli’s gland) secretion, which is clear and watery. The function of this secretion accordingly still requires explanation; according to Looss it serves as a covering secretion for the egg-shell proper. It appears also that other granules, the yolk granules as distinct from the shell drop granules, are not always used up during the development of the embryo and hence do not function as yolk, so these also when they exist, and frequently they are wanting, must serve some other purpose, possibly that of imbibing water for the use of the embryo.--J. W. W. S.]