In the chorion of the cockroach the surface appears to be finely granular, the finest granules being arranged in large, more or less regularly hexagonal areas, which are bounded by narrow, dark spaces, containing somewhat larger though less dense granules. The surface of the eggs of certain Phasmids are variously sculptured (Fig. 494).
The true structure of the chorion can only be, as Wheeler observes, seen in cross-sections, as shown by Blochmann, and also by Wheeler. The chorion consists of two chitinous laminæ kept in close apposition by means of numerous minute trabeculæ or pillars. It is the ends of these pillars that look like granules. In the spaces between the hexagonal areas, the trabeculæ are more scattered and individually thicker than those of the hexagons.
Fig. 495.—Egg of cotton-worm moth, Aletia: a, top view, showing the micropyle.—After Comstock.
Fig. 496.—Egg of Danais archippus.—After Riley.
These markings are of singular beauty and complexity in the eggs of many Lepidoptera, whose ova are variously ribbed, forming a beautiful fretwork of raised lines (Figs. 495 and 496), while in the Diptera and Hymenoptera the chorion is less solid, and usually smooth under low powers. The exochorion of the egg of the house and meat fly (C. vomitoria) is pitted with elongated hexagonal depressions, which cross the egg transversely. That of the honey-bee is also divided into long hexagonal areas (Fig. 497).
Fig. 497.—Egg with embryo of honey-bee, × 40: ch, chorion; ga, ganglia; s. ga, brain; jm, jaw-muscles forming; c, œsophageal collar; fb, fore intestine; mb, mid-intestine; ab, hind-intestine.—After Cheshire.
