Their general shape is spherical or oval, often cylindrical; where the eggs are long and cylindrical a dorsal and ventral side can be distinguished (Fig. 502). They are in the Tortricidæ and Limacodid moths flattened, thin, and scale-like. In the eggs of locusts and grasshoppers, as well as certain Diptera, the ventral side of the embryo corresponds to the convex side, and the concave side of the egg to the dorsal region of the embryo (Figs. 502 and 493).
There is an anterior and posterior end or pole, the anterior end being that which in the body of the parent lies towards her head, or towards the upper or distal end of the ovarian tube. Towards this end lies in the later stages of embryonic life the head-end of the embryo, while the posterior end of the embryo is turned towards the hinder pole of the egg (Figs. 493 and 520).
The egg-shell and yolk-membrane.—The ripe egg is protected by two membranes: 1, an inner or vitelline membrane or oölemma (dh) (Fig. 500, d), produced in the egg by a hardening of the outer layer, and 2, the outer or chorion (c), which is secreted by the cells of the ovarian follicle. The latter is divided into two layers: an inner, the endochorion, and an outer, the exochorion.
Fig. 503.—Fertilization of the egg of a round-worm (Ascaris megalocephala): A, the ends (centrosomes) of the spindle formed. B, the spindle completed; sp, sperm-nucleus, with its chromosomes; ei, egg-nucleus; p, polar bodies.—After Boveri, from Field’s Hertwig.
Fig. 493.—Eggs of Corixa: A, early stage before formation of the embryo, from one side. B, the same viewed in the plane of symmetry. C, the embryo in its final position; a, anterior, p, posterior, end; l, left, r, right, v, ventral, d, dorsal, aspect. (The letters refer to the final position of the embryo, which is nearly diametrically opposite to that in which it first develops); m, micropyle; p, pedicle.—After Metschnikoff, from Wilson.
Fig. 494.—Eggs of Phasmidæ: A, Lonchodes duivenbodi. B, Platycrania edulis. C, Haplopus grayi. D, Phyllium siccifolium.—After Kaup, from Sharp.
While the yolk-membrane is usually a completely homogeneous, thin, structureless membrane, the chorion or shell of the egg is usually covered with a network of ridges enclosing polygonal areas, varying in shape according to the species or genus. These external markings are due to the impress of the cellular structure of the epithelium of the ovarian follicle.