Dr. Holló Ferenc - Dr. Murai Éva szerk.: Parasitologia Hungarica 5. (Budapest, 1972)

the joining level of the 2-3 epidermal cell rows, begin their activity. The formation of the germinal tissue, situated in the posterior part of the embryo, takes place also during this period . During this time basic changes can be followed also in the vitelline cells, supplying the embryo with nutritive material and forming the envelope.When the embryo reaches a size of 50-60 /a, the grain quantity of the adjacent vitelline cells reduces intensely and their contens give an impression of a homogeneous viscous material; around the embryo a "network" develops from large /30x15 /u/

, square cells /photos 11, 13/

. With the growth of the embryo the number of vitelline cells further decreases, the discharge of grains involves also the peripherial cells, and the place cf the vitelline cells embracing the embryo is now occupied by two large vacuola and the so-called "mucoid plug" /BARLOW, 1925/. The mucoid plug could be observed in every case, being typically 25-40 in length and situated symmetrically or slightly asymmetrically /photo 14

/. On the yth and 10th days, the embryo reaches a size of 120-150 yu, and seems to be papable of hatching /photo 14/

. Hatching begins, however, only later, when the miracidium becomes not only morphologically but also physiologically capable of hatching and further development. The hatching is rather complicated, a process not yet cleared in details. The hatching rate of 3 groups of 1000 eggs, each collected from one worm-population and incubated under the same conditions, is shown in fig. 1. The graphs are summarized in fig. 2. The structure of the miracidium The hatching and free-swimming miracidium is fusiform or shellshaped, without eye spot and covered with.cilia /photo 12/

, an animacule of 155-225 u in length and 34-55 yu, in width,in a fixed state 110-140 /u, and 37-60 ju,, resp. At the apical end of the U