As the yolk cells grow older refracting spherules become deposited in their protoplasm, which either completely hide the nucleus, or render it very difficult to see. In the majority of cases the entire cells forming the lining of the vesicles constitute the secretion of the yolk-gland. They invest the ovum, and around them is formed a shell or membrane. In some cases (_e.g._ Polystomum integerrimum) the yolk cells retain their cellular character and vitality till the embryo is far developed. In other cases they lose their membrane and nucleus shortly after the formation of the egg-shell, and break up into a fluid, holding in suspension a number of yolk granules. A partial disorganisation of the yolk cells can also take place before they surround the ovum; while in some species of Distomum they completely break up before leaving the yolk-gland. [FIG. 9. GENERATIVE SYSTEM OF VORTEX VIRIDIS. (From Gegenbaur, after Max Schultze.) _t._ Testis. _v.d._ Vasa differentia. _v.s._ Seminal vesicle. _p._ Penis. _u._ Uterus. _o._ Ovary. _v._ Vagina. _g.v._ Yolk-glands. _r.s._ Receptaculum seminis.] There is thus a complete series of gradations between the investment of the ovum by a number of distinct cells, and its investment by a layer of fluid containing yolk-spherules in suspension. In neither the one case nor the other do the investing structures take any share in the direct formation of the embryo from the ovum. Physiologically speaking they play the same part as the white in the fowl's egg. The egg-shell, which is usually formed by a secretion of a special shell-gland opening into the oviduct, exhibits one or two peculiarities in the different species of Trematodes. In Amphistomum subclavatum it presents at one extremity a thickened area, which is pierced by a narrow micropyle. In other cases one extremity of the egg-shell is produced into a long process, and sometimes even both extremities are armed in this way. Opercula and other types of armature are also found in different forms. The mode of development of the ovum in Cestodes is very nearly the same as in Trematodes. The ovum becomes enveloped in the usual secretion of the yolk-gland; and an egg-shell is always formed by the secretion of a special shell-gland. Amongst the Turbellarians and Nemertines, there are greater variations in the arrangement of the female generative glands, than in the preceding types. In most of the Rhabdocoela and fresh-water Dendrocoela these organs resemble in their fundamental characters those of the Trematodes and Cestodes. There are present a paired or single ovary and a paired yolk-gland. The general arrangement of the organs is shewn in fig. 9. The blind end of the ovaries is usually (Ed. van Beneden, etc.) stated to be formed of a polynuclear protoplasmic basis, but Hallez (No. 10) has recently insisted that, even at the extreme end of the ovary, the germinal cells are quite distinct, and not confounded together.