DICYEMIDÆ AND ORTHONECTIDÆ. DICYEMIDÆ. The structure and development of these remarkable parasites in the renal organs of the Cephalopoda have recently been greatly elucidated by the researches of E. van Beneden; and although a male element has not been discovered, yet the embryos originate from bodies which have a close similarity to ordinary ova. Van Beneden has shewn that Dicyema consists in the adult state of (1) a single layer of ciliated epiblast cells, somewhat modified anteriorly to form a cephalic enlargement; and of (2) one large nucleated hypoblast cell enclosed within the epiblast. There are two kinds of embryo, both developed from germs which originate in the hypoblast cell. The two kinds of embryo arise in individuals of somewhat different forms. The one kind, called by Van Beneden the vermiform embryo, arises in the more elongated and thinner examples of Dicyema which have been named Nematogens. These embryos pass directly into the parent form without metamorphosis. The second kind of embryo, called infusoriform, is very different from the parent, and has a free existence. Its eventual history is not known. It originates in the shorter and thicker individuals of Dicyema; which have been called Rhombogens. The Vermiform Embryos. The germs or cells which give rise to the vermiform embryos originate endogenously in the protoplasmic reticulum of the axial hypoblast cell. They appear as small but well-defined spheres, with a minute body in the centre. In these spheres a cortical layer becomes differentiated, which gradually increases in thickness and gives rise to the body of a cell, the nucleus and nucleolus of which are respectively formed from the inner part of the original sphere and the minute central body. These germs can originate in all parts of the hypoblast cell and are frequently very numerous. [FIG. 61. A. GASTRULA STAGE OF DICYEMA TYPUS. B. VERIFORM EMBRYO OF DICYEMA TYPUS. (From Gegenbaur, after E. van Beneden.)] The germ when completely formed undergoes a segmentation very similar to that of an ordinary ovum. It divides first into two and then into four approximately equal segments. Of the four segments one, however, remains passive for the remainder of the development. The other three divide and arrange themselves so as partially to enclose in a cup-like fashion the passive cell (fig. 61 A). The six cells resulting from their division again divide, giving rise to twelve cells, which nearly enclose the passive cell, leaving only a small aperture at one point. The whole process by which the central cell becomes enclosed is, as E. van Beneden points out, identical with a gastrula formation by epibole, and the space where the central cell is left uncovered is the blastopore. The central cell itself gives origin to the hypoblast cell of the adult, and the peripheral cells to the epiblast.