The ovary has a sack-like form and contains in some instances a central lumen (Polystomum integerrimum). At the blind end of the organ is placed the germinal tissue. This part is, according to the accounts of the majority of investigators, formed of a polynuclear mass of protoplasm not divided into distinct cells. Whether it is really formed of undivided protoplasm or not, it is quite certain that a little lower down in the organ distinct cells are found, which have been segmented off from the above mass, and are formed of a large nucleus and nucleolus, surrounded by a delicate layer of protoplasm. These cells are the young ova. They usually assume a more or less angular form from mutual pressure, and, in the cases where the ovary has a lumen, constitute a kind of epithelial lining for the ovarian tube. They become successively larger in passing down the ovary, and, though in most cases naked, are in some instances (Polystomum integerrimum) invested by a delicate vitelline membrane. Eventually the ova pass into the oviduct and become free and at the same time assume a spherical form.
In the oviduct the ovum receives somewhat remarkable investing structures, derived from the organ before spoken of as the yolk-gland. The yolk-gland consists of a number of small vesicles, each provided with a special duct, connected with the main duct of the gland. Each vesicle is lined by an epithelium of cells provided with doubly contoured membranes, and containing nuclei.
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.