Self-impregnation appears to occur as well as reciprocal impregnation2. The ovum has a shell at first clear, then becoming coloured, furnished at one end, except in Bilharzia, with a cover or operculum which the mature embryo thrusts open when it escapes. It is usually oval and smooth in the digenetic Trematoda. But in the monogenetic its shape varies, and is determined by that of the 'ootype,' and it is generally provided with a long process at one pole, or with two at opposite poles, and it is often fixed. In the monogenetic Trematoda it is always laid; in the digenetic it may, or may not commence its development in the oviduct. If development begins, it may be completed before oviposition as in Monostomum mutabile, or only the first stages of fission may be passed through. The germ-cell Usually lies near the pole, closed by the operculum, entirely or partially immersed among the vitelline cells. Fission is regular or irregular, but complete, and during its progress the vitelline cells degenerate and become used up more or less completely by the developing germ-cell. It has been found in those cases which have been thoroughly investigated that two layers of cells are successively differentiated from the surface of the embryonic mass (Schauinsland). The outermost layer is formed as follows.

A cell at the upper, i. e. opercular pole of the embryonic mass becomes flattened, divides, and the two cells then grow round the mass; other cells appear by division or by differentiation from the mass itself. Eventually an enveloping membrane is formed which lines the shell. A second superficial layer of flattened cells is next differentiated, which becomes a ciliated coat, or when cilia are not developed, a structureless cuticle as in D. tereticolle. The embryo of the monogenetic Trematoda appears to be non-ciliated as a rule. However, that of Diplozoon (= Diporpd) is ciliated laterally, of Polystomum integerrimum provided with five transverse ciliated bands, three anterior, incomplete dorsally, two posterior, incomplete ventrally. Among the digenetic genera some are, and some are not ciliated, and the cilia may cover the embryo but partially, e. g. they are present only on the anterior region of D. lanceolatum. The ciliated coat or its representative, is subsequently lost, but an underlying layer of flattened cells, afterwards transformed into the cuticle, covers the embryo. A caecal digestive tract is formed anteriorly, and the pharynx is sometimes to' be detected. Flame-cells are also present, e.g. as a single pair in the embryo of Fasciola hepatica.

One or two black eye-specks with or without a lens-like body may be developed. And the embryo may be furnished at its anterior extremity, either with chitinoid plates or spines as in D. tereticolle, or an extensile boring process as in F. hepatica. When it quits the egg-shell it bursts the enveloping membrane, and then thrusts open the operculum. It may at integerrimum. Its existence has been repeatedly denied. Ijima has recently maintained that Zeller's canal also exists in P. ocellatum, in Diplozoon, and an Octobothrium (sp. ?), but that it communicates with the alimentary canal in which he has observed germs in P. integerrimum. See Z. A. vii. 1883, p. 635.

1 The prematurely sexual form of Polystomum integerrimum has sexual organs simplified as compared with those of the usual form. The testis is single, globular; and the vas deferens without prostatic glands. The germarium is long, coiled; the vitellarium of restricted extent; the two Laurer-Stieda canals absent; the oviduct devoid of a terminal coiled and dilated portion. Each ovum is laid as soon as formed. The animal is probably self-fertilising. Zeller, Z. W. Z. xxvii. 1876.

2Reciprocal impregnation has been observed by Zeller in P. integerrimum, by Looss in D. clavigerum (Z. W. Z. xli. 1885, p. 426), by Cobbold in Campula ( = D. campula). The presence of hooks round the female apertures of some monogenetic Trematoda points in the same direction. In Bilharzia, where the sexes are separate, the male is so curled as to form a 'gynaecophoric' canal for the female.

Self-impregnation occurs beyond a doubt in some instances. A single P. integerrimum has been found in a Frog's bladder with sperm in the female passages. Von Linstow met with in Gammarus Pulex a Distome (D. agamos) encysted, a single Distome in each cyst. Some individuals were sexless; others had mature male organs; others again mature female organs. Developing ova were present in the oviduct. In this instance the male aperture is behind, the female in front of the ventral sucker. Sometimes the structure of the apertures makes self-impregnation more than probable. See Poirier, A. Z. Expt. (2), iii. 1885, pp. 582-85. Zaddach has actually observed the act in D. cirrigerum (Z. A. iv. 1881, p. 427), encysted in Astacus. The ova of this animal are laid in the cyst; the parent dies; the cyst decays, and the ova are then scattered. Leuckart states that he has noticed sexual maturity attained in cases of prolonged encystation in a host, e. g. in Ephemerae larvae (Parasiten, ed. 2, i. note, p. 98). For a discussion of the whole subject, see Looss, Z. W. Z. xli. pp. 420-27.

The same moment moult its ciliated coat (D. cylinidraceumi (?) D. mentula-tum), but this coat is not usually lost until the embryo enters its first host.

A canal has often been described as connecting the testis to the germ-duct, e. g. by Zeller in P.