In most Ectoprocta, however, the development takes place internally or in an ovicell, and a considerable quantity of yolk is present.
The relative size of the eggs and the amount of nutritive yolk which they contain are generally much greater in those forms which have a direct development.
Maculosa, but as a rule only one fully develops in each uterus, the embryo being nourished on the yolk of the other eggs, which more or less dissolve to form a large mass of nutrient matter.
The hypoblast is formed either by a definite invagination or by the immigration of isolated cells, known as vitellophags, which wander through the yolk and later become associated into a definite mesenteron, or by some combination of these two methods.
Wheeler, the amnion is ruptured and turned back from covering the germ band, enclosing the yolk dorsally and becoming finally absorbed, as the ectoderm of the germ band itself spreads to form the dorsal wall.
Means of conveying superfluous yolk to the gut, where it may serve as food.
The germ band evidently marks the ventral aspect of the developing insect, whose body must be completed by the extension of the embryo so as to enclose the yolk dorsally.
Ov., ovaries; sh.g., shell gland; y.g., yolk gland; r.s., receptaculum seminis; ut., uterus; X 7.
F, Foot; op, operculum; mn, anal papilla; ry, dry, two portions of unabsorbed nutritive yolk on either side of the intestine.
From the oviduct a long duct full of yolk passes backwards almost to the hinder end of the body and ends blindly in a globular dilatation just below the skin.