Fig. 544.—Cross-section through the abdominal region of a somewhat older primitive band of Phyllodromia germanica: bg, rudiment of the nerve-cord; c, remains of the cœlomic cavity; cz, rudiment of the genital efferent passage; ec, ectoderm; en, endoderm; ef, terminal cord-plate; fk, fat-body tissue; gz, genital cells; h, rudiment of the heart; p, rudiment of the pericardial cavity; ps, rudiment of the pericardial septum; so, somatic mesoderm layer; sp, splanchnic mesoderm layer.
The salivary glands.—These segmentally arranged glands, which open by pairs into the three gnathal segments of the head, arise as ectodermal invaginations originally opening not into the stomodæum, but outwards on the surface of the body; hence Korschelt and Heider suggest that they were originally dermal glands, whose mouths became drawn into the buccal cavity.
Fig. 545.—Cross-section through the abdominal region of an embryo of cockroach (P. germanica) after the yolk has been completely enclosed by the primitive band and the closure of the back; s, tracheal stigma; other letters as in Figs. 540, 544.—This and Fig. 544 after Heymons, from Korschelt and Heider.
Fig. 546.—Embryo of Doryphora shortly after the appearance of the appendages, unrolled and isolated: o, stomodæum; lb, labrum; b1–b3, three brain segments; og1–og3, three segments of the optic ganglion; op1–op3, three segments of the optic plate; f1–f5, five pairs of invaginations which form the tentorium, etc.; t7–t20, tracheal invaginations; the two last pairs (t19-t20) either disappear or form the openings of the sexual ducts; at, antennæ; md, mandibles; mx1–mx2, maxillæ: p1–p3, legs; c, commissure connecting the two ganglionic thickenings (g4) of the premandibular segment; gl, ganglia; mst, middlecord thickenings; mpg1–mpg3, rudiments of three pairs of urinary tubes; a, proctodæum.—After Wheeler.
For their serial arrangement, see p. 337. Korschelt and Heider state that they would be inclined to homologize the salivary glands of insects with those glands of myriopods opening into the mouth-cavity, were it not that these glands in myriopods opening into the mouth are in reality transformed nephridia originating from the mesoderm, while the salivary glands of insects are clearly ectodermal structures. We must, therefore, they add, leave to later researches the question of the homology of these organs, also of their relations to the similar glands of Peripatus.
Fig. 547.—Section of proctodæum of embryo locust, showing origin of urinary tubes (ur.t): ep, epithelial or glandular layer; m, cells of outer or muscular layer; a, section of a tube.