(_b_) _Eggs_ (Figs. 143, 146 and 147).—Each egg is composed of the following parts: (1) A true germ cell, which originates in the ovary and is destined to give rise to the future embryo; (2) a number of vitelline or yolk cells, which are formed in a specialised and independent portion (vitellogene gland) of the female glands—instead of developing into embryos the yolk cells form a follicle-like covering for the true germ cell, and play an important _rôle_ in the nutrition of the latter as it undergoes further development; (3) a shell surrounding the germ cell and vitelline cells, and provided at one end with a cap or operculum. The eggs escape from the uterus of the adult through the vulva, are carried to the intestine of the host with the bile, then pass through the intestines with the contents of the latter, and are expelled from the host with the fæcal matter. Many of them become dried, and then undergo no further development; but others are naturally dropped in the water in marshes, or, being dropped on dry ground, they are washed into water by the rain, or are carried to a more favourable position by the feet of animals pasturing or passing through the fields. After a longer or shorter period of incubation, which varies with the temperature, a ciliated embryo (_miracidium_) is developed. At a temperature of 20° to 26° C. the miracidium may be formed in ten days to three weeks; at a temperature of 16° C. the development takes two to three months; at 38° C. it ceases entirely. Experiments have shown that as long as these eggs remain in the dark the miracidium will not escape from the egg-shell; accordingly it will not escape during the night. When exposed to the light, however, or when suddenly brought into contact with cold water, the organism bursts the cap from the egg-shell, crawls through the opening, and becomes a— [Illustration: $1] [Illustration: $1] [Illustration: $1] (_c_) _Free-swimming ciliated miracidium_ (Fig. 148).—As already stated, this organism is entirely different from its mother. It measures about 0·15 mm. long; is somewhat broader in its anterior portion than in its posterior portion; on its anterior extremity we find a small eminence, known as a boring papilla; the exterior surface of the young worm is covered with numerous cilia, which by their motion propel the animal through the water; inside the body we find in the anterior portion a simple vestigial intestine and a double ganglionic mass provided with a peculiar pigmented double cup-shaped eye-spot; in the posterior portion of the body cavity are found a number of germ cells, which develop into individuals of the next generation. [Illustration: $1] [Illustration: $1] [Illustration: $1]